Re: [PATCH v22 02/18] mm/damon: Implement region based sampling

[Shakeel Butt]

On Tue, Oct 20, 2020 at 2:01 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote:
>
> From: SeongJae Park <sjpark@xxxxxxxxx>
>
> DAMON separates its monitoring target address space independent high
> level logics

Please rewrite the above sentence to be more clear.

> from the target space dependent low level primitives for
> flexible support of various address spaces.
>
> This commit implements DAMON's target address space independent high
> level logics for basic access check and region based sampling.

Same for the above text.

> Hence,
> without the target address space specific parts implementations, this
> doesn't work alone.  A reference implementation of those will be
> provided by a later commit.
>
> Basic Access Check
> ==================
>
> The output of DAMON says what pages are how frequently accessed for a
> given duration.  The resolution of the access frequency is controlled by
> setting ``sampling interval`` and ``aggregation interval``.  In detail,
> DAMON checks access to each page per ``sampling interval`` and
> aggregates the results.  In other words, counts the number of the
> accesses to each page.  After each ``aggregation interval`` passes,
> DAMON calls callback functions that previously registered by users so
> that users can read the aggregated results and then clears the results.
> This can be described in below simple pseudo-code::
>
>     while monitoring_on:
>         for page in monitoring_target:
>             if accessed(page):
>                 nr_accesses[page] += 1
>         if time() % aggregation_interval == 0:
>             for callback in user_registered_callbacks:
>                 callback(monitoring_target, nr_accesses)
>             for page in monitoring_target:
>                 nr_accesses[page] = 0
>         sleep(sampling interval)
>
> The monitoring overhead of this mechanism will arbitrarily increase as
> the size of the target workload grows.
>

Please move the sampling part in a separate patch.

> Region Based Sampling
> =====================
>
> To avoid the unbounded increase of the overhead, DAMON groups adjacent
> pages that assumed to have the same access frequencies into a region.
> As long as the assumption (pages in a region have the same access
> frequencies) is kept, only one page in the region is required to be
> checked.  Thus, for each ``sampling interval``, DAMON randomly picks one
> page in each region, waits for one ``sampling interval``, checks whether
> the page is accessed meanwhile, and increases the access frequency of
> the region if so.  Therefore, the monitoring overhead is controllable by
> setting the number of regions.  DAMON allows users to set the minimum
> and the maximum number of regions for the trade-off.
>
> This scheme, however, cannot preserve the quality of the output if the
> assumption is not guaranteed.  Next commit will address this problem.
>
> Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> Reviewed-by: Leonard Foerster <foersleo@xxxxxxxxx>
> ---
>  include/linux/damon.h | 133 ++++++++++++++++-
>  mm/damon/core.c       | 333 ++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 465 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index 183e0edd7f43..1f7b095646c2 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -8,6 +8,8 @@
>  #ifndef _DAMON_H_
>  #define _DAMON_H_
>
> +#include <linux/mutex.h>
> +#include <linux/time64.h>
>  #include <linux/types.h>
>
>  /**
> @@ -23,11 +25,13 @@ struct damon_addr_range {
>  /**
>   * struct damon_region - Represents a monitoring target region.
>   * @ar:                        The address range of the region.
> + * @sampling_addr:     Address of the sample for the next access check.
>   * @nr_accesses:       Access frequency of this region.
>   * @list:              List head for siblings.
>   */
>  struct damon_region {
>         struct damon_addr_range ar;
> +       unsigned long sampling_addr;
>         unsigned int nr_accesses;
>         struct list_head list;
>  };
> @@ -50,12 +54,130 @@ struct damon_target {
>         struct list_head list;
>  };
>
> +struct damon_ctx;
> +
>  /**
> - * struct damon_ctx - Represents a context for each monitoring.
> + * struct damon_primitive      Monitoring primitives for given use cases.
> + *
> + * @init_target_regions:       Constructs initial monitoring target regions.
> + * @prepare_access_checks:     Prepares next access check of target regions.
> + * @check_accesses:            Checks the access of target regions.
> + * @target_valid:              Determine if the target is valid.
> + * @cleanup:                   Cleans up the context.
> + *
> + * DAMON can be extended for various address spaces and usages.  For this,
> + * users should register the low level primitives for their target address
> + * space and usecase via the &damon_ctx.primitive.  Then, the monitoring thread
> + * calls @init_target_regions before starting the monitoring and
> + * @prepare_access_checks, @check_accesses, and @target_valid for each
> + * @sample_interval.
> + *
> + * @init_target_regions should construct proper monitoring target regions and
> + * link those to the DAMON context struct.
> + * @prepare_access_checks should manipulate the monitoring regions to be
> + * prepare for the next access check.
> + * @check_accesses should check the accesses to each region that made after the
> + * last preparation and update the `->nr_accesses` of each region.  It should
> + * also return max &damon_region.nr_accesses that made as a result of its
> + * update.
> + * @target_valid should check whether the target is still valid for the
> + * monitoring.
> + * @cleanup is called from @kdamond just before its termination.  After this
> + * call, only @kdamond_lock and @kdamond will be touched.
> + */
> +struct damon_primitive {
> +       void (*init_target_regions)(struct damon_ctx *context);
> +       void (*prepare_access_checks)(struct damon_ctx *context);
> +       unsigned int (*check_accesses)(struct damon_ctx *context);
> +       bool (*target_valid)(struct damon_target *target);
> +       void (*cleanup)(struct damon_ctx *context);
> +};
> +
> +/*
> + * struct damon_callback       Monitoring events notification callbacks.
> + *
> + * @before_start:      Called before starting the monitoring.
> + * @after_sampling:    Called after each sampling.
> + * @after_aggregation: Called after each aggregation.
> + * @before_terminate:  Called before terminating the monitoring.
> + * @private:           User private data.
> + *
> + * The monitoring thread (&damon_ctx->kdamond) calls @before_start and
> + * @before_terminate just before starting the monitoring and just before
> + * finishing the monitoring.  Therefore, those are good places for installing
> + * and cleaning @private.
> + *
> + * The monitoring thread calls @after_sampling and @after_aggregation for each
> + * of the sampling intervals and aggregation intervals, respectively.
> + * Therefore, users can safely access the monitoring results via
> + * &damon_ctx.targets_list without additional protection of
> + * damon_ctx.kdamond_lock.  For the reason, users are recommended to use these
> + * callback for the accesses to the results.
> + *
> + * If any callback returns non-zero, monitoring stops.
> + */
> +struct damon_callback {
> +       void *private;
> +
> +       int (*before_start)(struct damon_ctx *context);
> +       int (*after_sampling)(struct damon_ctx *context);
> +       int (*after_aggregation)(struct damon_ctx *context);
> +       int (*before_terminate)(struct damon_ctx *context);
> +};
> +
> +/**
> + * struct damon_ctx - Represents a context for each monitoring.  This is the
> + * main interface that allows users to set the attributes and get the results
> + * of the monitoring.
> + *
> + * @sample_interval:           The time between access samplings.
> + * @aggr_interval:             The time between monitor results aggregations.
> + * @nr_regions:                        The number of monitoring regions.
> + *
> + * For each @sample_interval, DAMON checks whether each region is accessed or
> + * not.  It aggregates and keeps the access information (number of accesses to
> + * each region) for @aggr_interval time.  All time intervals are in
> + * micro-seconds.
> + *
> + * @kdamond:           Kernel thread who does the monitoring.
> + * @kdamond_stop:      Notifies whether kdamond should stop.
> + * @kdamond_lock:      Mutex for the synchronizations with @kdamond.
> + *
> + * For each monitoring context, one kernel thread for the monitoring is
> + * created.  The pointer to the thread is stored in @kdamond.
> + *
> + * Once started, the monitoring thread runs until explicitly required to be
> + * terminated or every monitoring target is invalid.  The validity of the
> + * targets is checked via the @target_valid callback.  The termination can also
> + * be explicitly requested by writing non-zero to @kdamond_stop.  The thread
> + * sets @kdamond to NULL when it terminates.  Therefore, users can know whether
> + * the monitoring is ongoing or terminated by reading @kdamond.  Reads and
> + * writes to @kdamond and @kdamond_stop from outside of the monitoring thread
> + * must be protected by @kdamond_lock.
> + *
> + * Note that the monitoring thread protects only @kdamond and @kdamond_stop via
> + * @kdamond_lock.  Accesses to other fields must be protected by themselves.
> + *
>   * @targets_list:      Head of monitoring targets (&damon_target) list.
> + *
> + * @primitive: Set of monitoring primitives for given use cases.
> + * @callback:  Set of callbacks for monitoring events notifications.
>   */
>  struct damon_ctx {
> +       unsigned long sample_interval;
> +       unsigned long aggr_interval;
> +       unsigned long nr_regions;

IMO region is the property of the target being monitored. There might
be multiple types of target which might need the same abstraction like
damon_region and damon_addr_range but that sharing can be done at that
time.

> +
> +       struct timespec64 last_aggregation;
> +
> +       struct task_struct *kdamond;
> +       bool kdamond_stop;
> +       struct mutex kdamond_lock;
> +
>         struct list_head targets_list;  /* 'damon_target' objects */
> +
> +       struct damon_primitive primitive;
> +       struct damon_callback callback;
>  };
>
>  #define damon_next_region(r) \
> @@ -90,6 +212,15 @@ void damon_free_target(struct damon_target *t);
>  void damon_destroy_target(struct damon_target *t);
>  unsigned int damon_nr_regions(struct damon_target *t);
>
> +int damon_set_targets(struct damon_ctx *ctx,
> +               unsigned long *ids, ssize_t nr_ids);
> +int damon_set_attrs(struct damon_ctx *ctx, unsigned long sample_int,
> +               unsigned long aggr_int, unsigned long nr_reg);
> +
> +int damon_nr_running_ctxs(void);
> +int damon_start(struct damon_ctx **ctxs, int nr_ctxs);
> +int damon_stop(struct damon_ctx **ctxs, int nr_ctxs);
> +
>  #endif /* CONFIG_DAMON */
>
>  #endif
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 4562b2458719..eb4ebeaa064d 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -8,12 +8,20 @@
>  #define pr_fmt(fmt) "damon: " fmt
>
>  #include <linux/damon.h>
> +#include <linux/delay.h>
> +#include <linux/kthread.h>
>  #include <linux/slab.h>
>
> +/* Minimal region size.  Every damon_region is aligned by this. */
> +#define MIN_REGION PAGE_SIZE
> +
>  /*
>   * Functions and macros for DAMON data structures
>   */
>
> +static DEFINE_MUTEX(damon_lock);
> +static int nr_running_ctxs;
> +
>  /*
>   * Construct a damon_region struct
>   *
> @@ -119,3 +127,328 @@ unsigned int damon_nr_regions(struct damon_target *t)
>
>         return nr_regions;
>  }
> +
> +/**
> + * damon_set_targets() - Set monitoring targets.
> + * @ctx:       monitoring context
> + * @ids:       array of target ids
> + * @nr_ids:    number of entries in @ids
> + *
> + * This function should not be called while the kdamond is running.
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +int damon_set_targets(struct damon_ctx *ctx,
> +                     unsigned long *ids, ssize_t nr_ids)
> +{
> +       ssize_t i;
> +       struct damon_target *t, *next;
> +
> +       damon_for_each_target_safe(t, next, ctx)
> +               damon_destroy_target(t);
> +
> +       for (i = 0; i < nr_ids; i++) {
> +               t = damon_new_target(ids[i]);
> +               if (!t) {
> +                       pr_err("Failed to alloc damon_target\n");
> +                       return -ENOMEM;
> +               }
> +               damon_add_target(ctx, t);
> +       }
> +
> +       return 0;
> +}

This function looks weird and without usage. I suppose this is called
when a user requests to set up monitoring targets. I would change this
to add targets one by one. At the moment, it removes the older targets
and adds newer ones which can fail and will leave the context in a
weird state.

BTW this function should come in the patch which introduces the user interface.

> +
> +/**
> + * damon_set_attrs() - Set attributes for the monitoring.
> + * @ctx:               monitoring context
> + * @sample_int:                time interval between samplings
> + * @aggr_int:          time interval between aggregations
> + * @nr_reg:            number of regions
> + *
> + * This function should not be called while the kdamond is running.
> + * Every time interval is in micro-seconds.
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +int damon_set_attrs(struct damon_ctx *ctx, unsigned long sample_int,
> +                   unsigned long aggr_int, unsigned long nr_reg)
> +{
> +       if (nr_reg < 3) {
> +               pr_err("nr_regions (%lu) must be at least 3\n",
> +                               nr_reg);
> +               return -EINVAL;
> +       }
> +
> +       ctx->sample_interval = sample_int;
> +       ctx->aggr_interval = aggr_int;
> +       ctx->nr_regions = nr_reg;
> +
> +       return 0;
> +}
> +
> +static bool damon_kdamond_running(struct damon_ctx *ctx)
> +{
> +       bool running;
> +
> +       mutex_lock(&ctx->kdamond_lock);
> +       running = ctx->kdamond != NULL;
> +       mutex_unlock(&ctx->kdamond_lock);
> +
> +       return running;
> +}
> +
> +static int kdamond_fn(void *data);
> +
> +/*
> + * __damon_start() - Starts monitoring with given context.
> + * @ctx:       monitoring context
> + *
> + * This function should be called while damon_lock is hold.
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +static int __damon_start(struct damon_ctx *ctx)
> +{
> +       int err = -EBUSY;
> +
> +       mutex_lock(&ctx->kdamond_lock);
> +       if (!ctx->kdamond) {
> +               err = 0;
> +               ctx->kdamond_stop = false;
> +               ctx->kdamond = kthread_create(kdamond_fn, ctx, "kdamond.%d",
> +                               nr_running_ctxs);
> +               if (IS_ERR(ctx->kdamond))
> +                       err = PTR_ERR(ctx->kdamond);
> +               else
> +                       wake_up_process(ctx->kdamond);
> +       }
> +       mutex_unlock(&ctx->kdamond_lock);
> +
> +       return err;
> +}
> +
> +/**
> + * damon_start() - Starts the monitorings for a given group of contexts.
> + * @ctxs:      an array of the pointers for contexts to start monitoring
> + * @nr_ctxs:   size of @ctxs
> + *
> + * This function starts a group of monitoring threads for a group of monitoring
> + * contexts.  One thread per each context is created and run in parallel.  The
> + * caller should handle synchronization between the threads by itself.  If a
> + * group of threads that created by other 'damon_start()' call is currently
> + * running, this function does nothing but returns -EBUSY.
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +int damon_start(struct damon_ctx **ctxs, int nr_ctxs)
> +{
> +       int i;
> +       int err = 0;
> +
> +       mutex_lock(&damon_lock);
> +       if (nr_running_ctxs) {

What is this checking?

> +               mutex_unlock(&damon_lock);
> +               return -EBUSY;
> +       }
> +
> +       for (i = 0; i < nr_ctxs; i++) {
> +               err = __damon_start(ctxs[i]);
> +               if (err)
> +                       break;
> +               nr_running_ctxs++;
> +       }
> +       mutex_unlock(&damon_lock);
> +
> +       return err;
> +}
> +
> +/*
> + * __damon_stop() - Stops monitoring of given context.
> + * @ctx:       monitoring context
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +static int __damon_stop(struct damon_ctx *ctx)
> +{
> +       mutex_lock(&ctx->kdamond_lock);
> +       if (ctx->kdamond) {
> +               ctx->kdamond_stop = true;
> +               mutex_unlock(&ctx->kdamond_lock);
> +               while (damon_kdamond_running(ctx))
> +                       usleep_range(ctx->sample_interval,
> +                                       ctx->sample_interval * 2);
> +               return 0;
> +       }
> +       mutex_unlock(&ctx->kdamond_lock);
> +
> +       return -EPERM;
> +}
> +
> +/**
> + * damon_stop() - Stops the monitorings for a given group of contexts.
> + * @ctxs:      an array of the pointers for contexts to stop monitoring
> + * @nr_ctxs:   size of @ctxs
> + *
> + * Return: 0 on success, negative error code otherwise.
> + */
> +int damon_stop(struct damon_ctx **ctxs, int nr_ctxs)
> +{
> +       int i, err = 0;
> +
> +       for (i = 0; i < nr_ctxs; i++) {
> +               /* nr_running_ctxs is decremented in kdamond_fn */
> +               err = __damon_stop(ctxs[i]);
> +               if (err)
> +                       return err;
> +       }
> +
> +       return err;
> +}
> +
> +/*
> + * Functions for DAMON core logics
> + */
> +
> +/*
> + * damon_check_reset_time_interval() - Check if a time interval is elapsed.
> + * @baseline:  the time to check whether the interval has elapsed since
> + * @interval:  the time interval (microseconds)
> + *
> + * See whether the given time interval has passed since the given baseline
> + * time.  If so, it also updates the baseline to current time for next check.
> + *
> + * Return:     true if the time interval has passed, or false otherwise.
> + */
> +static bool damon_check_reset_time_interval(struct timespec64 *baseline,
> +               unsigned long interval)
> +{
> +       struct timespec64 now;
> +
> +       ktime_get_coarse_ts64(&now);
> +       if ((timespec64_to_ns(&now) - timespec64_to_ns(baseline)) <
> +                       interval * 1000)
> +               return false;
> +       *baseline = now;
> +       return true;
> +}
> +
> +/*
> + * Check whether it is time to flush the aggregated information
> + */
> +static bool kdamond_aggregate_interval_passed(struct damon_ctx *ctx)
> +{
> +       return damon_check_reset_time_interval(&ctx->last_aggregation,
> +                       ctx->aggr_interval);
> +}
> +
> +/*
> + * Reset the aggregated monitoring results ('nr_accesses' of each region).
> + */
> +static void kdamond_reset_aggregated(struct damon_ctx *c)
> +{
> +       struct damon_target *t;
> +
> +       damon_for_each_target(t, c) {
> +               struct damon_region *r;
> +
> +               damon_for_each_region(r, t)
> +                       r->nr_accesses = 0;
> +       }
> +}
> +
> +/*
> + * Check whether current monitoring should be stopped
> + *
> + * The monitoring is stopped when either the user requested to stop, or all
> + * monitoring targets are invalid.
> + *
> + * Returns true if need to stop current monitoring.
> + */
> +static bool kdamond_need_stop(struct damon_ctx *ctx)
> +{
> +       struct damon_target *t;
> +       bool stop;
> +
> +       mutex_lock(&ctx->kdamond_lock);
> +       stop = ctx->kdamond_stop;
> +       mutex_unlock(&ctx->kdamond_lock);
> +       if (stop)
> +               return true;
> +
> +       if (!ctx->primitive.target_valid)
> +               return false;
> +
> +       damon_for_each_target(t, ctx) {
> +               if (ctx->primitive.target_valid(t))
> +                       return false;
> +       }

Why check for valid target? Are we gonna create a new kthread when the
user set the new target?

> +
> +       return true;
> +}
> +
> +static void set_kdamond_stop(struct damon_ctx *ctx, bool stop)
> +{
> +       mutex_lock(&ctx->kdamond_lock);
> +       ctx->kdamond_stop = stop;
> +       mutex_unlock(&ctx->kdamond_lock);
> +}
> +
> +#define kdamond_call_prmt(ctx, fn)                     \
> +       do {                                            \
> +               if (ctx->primitive.fn)                  \
> +                       ctx->primitive.fn(ctx);         \
> +       } while (0)
> +
> +#define kdamond_callback(ctx, fn)                              \
> +       do {                                                    \
> +               if (ctx->callback.fn && ctx->callback.fn(ctx))  \
> +                       set_kdamond_stop(ctx, true);            \
> +       } while (0)

I don't think these macros are helpful rather they make code harder to
grasp. Also don't hide the function call like set_kdamond_stop() in a
macro.

> +
> +/*
> + * The monitoring daemon that runs as a kernel thread
> + */
> +static int kdamond_fn(void *data)
> +{
> +       struct damon_ctx *ctx = (struct damon_ctx *)data;
> +       struct damon_target *t;
> +       struct damon_region *r, *next;
> +
> +       pr_info("kdamond (%d) starts\n", ctx->kdamond->pid);
> +

Call the callbacks explicitly.


> +       kdamond_call_prmt(ctx, init_target_regions);
> +       kdamond_callback(ctx, before_start);
> +
> +       while (!kdamond_need_stop(ctx)) {
> +               kdamond_call_prmt(ctx, prepare_access_checks);
> +               kdamond_callback(ctx, after_sampling);
> +
> +               usleep_range(ctx->sample_interval, ctx->sample_interval + 1);
> +
> +               kdamond_call_prmt(ctx, check_accesses);
> +
> +               if (kdamond_aggregate_interval_passed(ctx)) {
> +                       kdamond_callback(ctx, after_aggregation);
> +                       kdamond_reset_aggregated(ctx);
> +               }
> +       }
> +       damon_for_each_target(t, ctx) {
> +               damon_for_each_region_safe(r, next, t)
> +                       damon_destroy_region(r);
> +       }
> +
> +       kdamond_callback(ctx, before_terminate);
> +       kdamond_call_prmt(ctx, cleanup);
> +
> +       pr_debug("kdamond (%d) finishes\n", ctx->kdamond->pid);
> +       mutex_lock(&ctx->kdamond_lock);
> +       ctx->kdamond = NULL;
> +       mutex_unlock(&ctx->kdamond_lock);
> +
> +       mutex_lock(&damon_lock);
> +       nr_running_ctxs--;
> +       mutex_unlock(&damon_lock);
> +
> +       do_exit(0);
> +}
> --
> 2.17.1
>