Re: [PATCH 5/5] writeback, memcg: Implement foreign dirty flushing

[Jan Kara]

On Mon 26-08-19 09:06:56, Tejun Heo wrote:
> There's an inherent mismatch between memcg and writeback.  The former
> trackes ownership per-page while the latter per-inode.  This was a
> deliberate design decision because honoring per-page ownership in the
> writeback path is complicated, may lead to higher CPU and IO overheads
> and deemed unnecessary given that write-sharing an inode across
> different cgroups isn't a common use-case.
> 
> Combined with inode majority-writer ownership switching, this works
> well enough in most cases but there are some pathological cases.  For
> example, let's say there are two cgroups A and B which keep writing to
> different but confined parts of the same inode.  B owns the inode and
> A's memory is limited far below B's.  A's dirty ratio can rise enough
> to trigger balance_dirty_pages() sleeps but B's can be low enough to
> avoid triggering background writeback.  A will be slowed down without
> a way to make writeback of the dirty pages happen.
> 
> This patch implements foreign dirty recording and foreign mechanism so
> that when a memcg encounters a condition as above it can trigger
> flushes on bdi_writebacks which can clean its pages.  Please see the
> comment on top of mem_cgroup_track_foreign_dirty_slowpath() for
> details.
> 
> A reproducer follows.
> 
> write-range.c::
> 
>   #include <stdio.h>
>   #include <stdlib.h>
>   #include <unistd.h>
>   #include <fcntl.h>
>   #include <sys/types.h>
> 
>   static const char *usage = "write-range FILE START SIZE\n";
> 
>   int main(int argc, char **argv)
>   {
> 	  int fd;
> 	  unsigned long start, size, end, pos;
> 	  char *endp;
> 	  char buf[4096];
> 
> 	  if (argc < 4) {
> 		  fprintf(stderr, usage);
> 		  return 1;
> 	  }
> 
> 	  fd = open(argv[1], O_WRONLY);
> 	  if (fd < 0) {
> 		  perror("open");
> 		  return 1;
> 	  }
> 
> 	  start = strtoul(argv[2], &endp, 0);
> 	  if (*endp != '\0') {
> 		  fprintf(stderr, usage);
> 		  return 1;
> 	  }
> 
> 	  size = strtoul(argv[3], &endp, 0);
> 	  if (*endp != '\0') {
> 		  fprintf(stderr, usage);
> 		  return 1;
> 	  }
> 
> 	  end = start + size;
> 
> 	  while (1) {
> 		  for (pos = start; pos < end; ) {
> 			  long bread, bwritten = 0;
> 
> 			  if (lseek(fd, pos, SEEK_SET) < 0) {
> 				  perror("lseek");
> 				  return 1;
> 			  }
> 
> 			  bread = read(0, buf, sizeof(buf) < end - pos ?
> 					       sizeof(buf) : end - pos);
> 			  if (bread < 0) {
> 				  perror("read");
> 				  return 1;
> 			  }
> 			  if (bread == 0)
> 				  return 0;
> 
> 			  while (bwritten < bread) {
> 				  long this;
> 
> 				  this = write(fd, buf + bwritten,
> 					       bread - bwritten);
> 				  if (this < 0) {
> 					  perror("write");
> 					  return 1;
> 				  }
> 
> 				  bwritten += this;
> 				  pos += bwritten;
> 			  }
> 		  }
> 	  }
>   }
> 
> repro.sh::
> 
>   #!/bin/bash
> 
>   set -e
>   set -x
> 
>   sysctl -w vm.dirty_expire_centisecs=300000
>   sysctl -w vm.dirty_writeback_centisecs=300000
>   sysctl -w vm.dirtytime_expire_seconds=300000
>   echo 3 > /proc/sys/vm/drop_caches
> 
>   TEST=/sys/fs/cgroup/test
>   A=$TEST/A
>   B=$TEST/B
> 
>   mkdir -p $A $B
>   echo "+memory +io" > $TEST/cgroup.subtree_control
>   echo $((1<<30)) > $A/memory.high
>   echo $((32<<30)) > $B/memory.high
> 
>   rm -f testfile
>   touch testfile
>   fallocate -l 4G testfile
> 
>   echo "Starting B"
> 
>   (echo $BASHPID > $B/cgroup.procs
>    pv -q --rate-limit 70M < /dev/urandom | ./write-range testfile $((2<<30)) $((2<<30))) &
> 
>   echo "Waiting 10s to ensure B claims the testfile inode"
>   sleep 5
>   sync
>   sleep 5
>   sync
>   echo "Starting A"
> 
>   (echo $BASHPID > $A/cgroup.procs
>    pv < /dev/urandom | ./write-range testfile 0 $((2<<30)))
> 
> v2: Added comments explaining why the specific intervals are being used.
> 
> v3: Use 0 @nr when calling cgroup_writeback_by_id() to use best-effort
>     flushing while avoding possible livelocks.
> 
> v4: Use get_jiffies_64() and time_before/after64() instead of raw
>     jiffies_64 and arthimetic comparisons as suggested by Jan.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  include/linux/backing-dev-defs.h |   1 +
>  include/linux/memcontrol.h       |  39 +++++++++
>  mm/memcontrol.c                  | 134 +++++++++++++++++++++++++++++++
>  mm/page-writeback.c              |   4 +
>  4 files changed, 178 insertions(+)
> 
> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> index 1075f2552cfc..4fc87dee005a 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -63,6 +63,7 @@ enum wb_reason {
>  	 * so it has a mismatch name.
>  	 */
>  	WB_REASON_FORKER_THREAD,
> +	WB_REASON_FOREIGN_FLUSH,
>  
>  	WB_REASON_MAX,
>  };
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 2cd4359cb38c..ad8f1a397ae4 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -183,6 +183,23 @@ struct memcg_padding {
>  #define MEMCG_PADDING(name)
>  #endif
>  
> +/*
> + * Remember four most recent foreign writebacks with dirty pages in this
> + * cgroup.  Inode sharing is expected to be uncommon and, even if we miss
> + * one in a given round, we're likely to catch it later if it keeps
> + * foreign-dirtying, so a fairly low count should be enough.
> + *
> + * See mem_cgroup_track_foreign_dirty_slowpath() for details.
> + */
> +#define MEMCG_CGWB_FRN_CNT	4
> +
> +struct memcg_cgwb_frn {
> +	u64 bdi_id;			/* bdi->id of the foreign inode */
> +	int memcg_id;			/* memcg->css.id of foreign inode */
> +	u64 at;				/* jiffies_64 at the time of dirtying */
> +	struct wb_completion done;	/* tracks in-flight foreign writebacks */
> +};
> +
>  /*
>   * The memory controller data structure. The memory controller controls both
>   * page cache and RSS per cgroup. We would eventually like to provide
> @@ -307,6 +324,7 @@ struct mem_cgroup {
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	struct list_head cgwb_list;
>  	struct wb_domain cgwb_domain;
> +	struct memcg_cgwb_frn cgwb_frn[MEMCG_CGWB_FRN_CNT];
>  #endif
>  
>  	/* List of events which userspace want to receive */
> @@ -1237,6 +1255,18 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>  			 unsigned long *pheadroom, unsigned long *pdirty,
>  			 unsigned long *pwriteback);
>  
> +void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
> +					     struct bdi_writeback *wb);
> +
> +static inline void mem_cgroup_track_foreign_dirty(struct page *page,
> +						  struct bdi_writeback *wb)
> +{
> +	if (unlikely(&page->mem_cgroup->css != wb->memcg_css))
> +		mem_cgroup_track_foreign_dirty_slowpath(page, wb);
> +}
> +
> +void mem_cgroup_flush_foreign(struct bdi_writeback *wb);
> +
>  #else	/* CONFIG_CGROUP_WRITEBACK */
>  
>  static inline struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
> @@ -1252,6 +1282,15 @@ static inline void mem_cgroup_wb_stats(struct bdi_writeback *wb,
>  {
>  }
>  
> +static inline void mem_cgroup_track_foreign_dirty(struct page *page,
> +						  struct bdi_writeback *wb)
> +{
> +}
> +
> +static inline void mem_cgroup_flush_foreign(struct bdi_writeback *wb)
> +{
> +}
> +
>  #endif	/* CONFIG_CGROUP_WRITEBACK */
>  
>  struct sock;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 26e2999af608..eb626a290d93 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -87,6 +87,10 @@ int do_swap_account __read_mostly;
>  #define do_swap_account		0
>  #endif
>  
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +static DECLARE_WAIT_QUEUE_HEAD(memcg_cgwb_frn_waitq);
> +#endif
> +
>  /* Whether legacy memory+swap accounting is active */
>  static bool do_memsw_account(void)
>  {
> @@ -4238,6 +4242,127 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>  	}
>  }
>  
> +/*
> + * Foreign dirty flushing
> + *
> + * There's an inherent mismatch between memcg and writeback.  The former
> + * trackes ownership per-page while the latter per-inode.  This was a
> + * deliberate design decision because honoring per-page ownership in the
> + * writeback path is complicated, may lead to higher CPU and IO overheads
> + * and deemed unnecessary given that write-sharing an inode across
> + * different cgroups isn't a common use-case.
> + *
> + * Combined with inode majority-writer ownership switching, this works well
> + * enough in most cases but there are some pathological cases.  For
> + * example, let's say there are two cgroups A and B which keep writing to
> + * different but confined parts of the same inode.  B owns the inode and
> + * A's memory is limited far below B's.  A's dirty ratio can rise enough to
> + * trigger balance_dirty_pages() sleeps but B's can be low enough to avoid
> + * triggering background writeback.  A will be slowed down without a way to
> + * make writeback of the dirty pages happen.
> + *
> + * Conditions like the above can lead to a cgroup getting repatedly and
> + * severely throttled after making some progress after each
> + * dirty_expire_interval while the underyling IO device is almost
> + * completely idle.
> + *
> + * Solving this problem completely requires matching the ownership tracking
> + * granularities between memcg and writeback in either direction.  However,
> + * the more egregious behaviors can be avoided by simply remembering the
> + * most recent foreign dirtying events and initiating remote flushes on
> + * them when local writeback isn't enough to keep the memory clean enough.
> + *
> + * The following two functions implement such mechanism.  When a foreign
> + * page - a page whose memcg and writeback ownerships don't match - is
> + * dirtied, mem_cgroup_track_foreign_dirty() records the inode owning
> + * bdi_writeback on the page owning memcg.  When balance_dirty_pages()
> + * decides that the memcg needs to sleep due to high dirty ratio, it calls
> + * mem_cgroup_flush_foreign() which queues writeback on the recorded
> + * foreign bdi_writebacks which haven't expired.  Both the numbers of
> + * recorded bdi_writebacks and concurrent in-flight foreign writebacks are
> + * limited to MEMCG_CGWB_FRN_CNT.
> + *
> + * The mechanism only remembers IDs and doesn't hold any object references.
> + * As being wrong occasionally doesn't matter, updates and accesses to the
> + * records are lockless and racy.
> + */
> +void mem_cgroup_track_foreign_dirty_slowpath(struct page *page,
> +					     struct bdi_writeback *wb)
> +{
> +	struct mem_cgroup *memcg = page->mem_cgroup;
> +	struct memcg_cgwb_frn *frn;
> +	u64 now = get_jiffies_64();
> +	u64 oldest_at = now;
> +	int oldest = -1;
> +	int i;
> +
> +	/*
> +	 * Pick the slot to use.  If there is already a slot for @wb, keep
> +	 * using it.  If not replace the oldest one which isn't being
> +	 * written out.
> +	 */
> +	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
> +		frn = &memcg->cgwb_frn[i];
> +		if (frn->bdi_id == wb->bdi->id &&
> +		    frn->memcg_id == wb->memcg_css->id)
> +			break;
> +		if (time_before64(frn->at, oldest_at) &&
> +		    atomic_read(&frn->done.cnt) == 1) {
> +			oldest = i;
> +			oldest_at = frn->at;
> +		}
> +	}
> +
> +	if (i < MEMCG_CGWB_FRN_CNT) {
> +		/*
> +		 * Re-using an existing one.  Update timestamp lazily to
> +		 * avoid making the cacheline hot.  We want them to be
> +		 * reasonably up-to-date and significantly shorter than
> +		 * dirty_expire_interval as that's what expires the record.
> +		 * Use the shorter of 1s and dirty_expire_interval / 8.
> +		 */
> +		unsigned long update_intv =
> +			min_t(unsigned long, HZ,
> +			      msecs_to_jiffies(dirty_expire_interval * 10) / 8);
> +
> +		if (time_before64(frn->at, now - update_intv))
> +			frn->at = now;
> +	} else if (oldest >= 0) {
> +		/* replace the oldest free one */
> +		frn = &memcg->cgwb_frn[oldest];
> +		frn->bdi_id = wb->bdi->id;
> +		frn->memcg_id = wb->memcg_css->id;
> +		frn->at = now;
> +	}
> +}
> +
> +/* issue foreign writeback flushes for recorded foreign dirtying events */
> +void mem_cgroup_flush_foreign(struct bdi_writeback *wb)
> +{
> +	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
> +	unsigned long intv = msecs_to_jiffies(dirty_expire_interval * 10);
> +	u64 now = jiffies_64;
> +	int i;
> +
> +	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
> +		struct memcg_cgwb_frn *frn = &memcg->cgwb_frn[i];
> +
> +		/*
> +		 * If the record is older than dirty_expire_interval,
> +		 * writeback on it has already started.  No need to kick it
> +		 * off again.  Also, don't start a new one if there's
> +		 * already one in flight.
> +		 */
> +		if (time_after64(frn->at, now - intv) &&
> +		    atomic_read(&frn->done.cnt) == 1) {
> +			frn->at = 0;
> +			cgroup_writeback_by_id(frn->bdi_id, frn->memcg_id, 0,
> +					       WB_REASON_FOREIGN_FLUSH,
> +					       &frn->done);
> +		}
> +	}
> +}
> +
>  #else	/* CONFIG_CGROUP_WRITEBACK */
>  
>  static int memcg_wb_domain_init(struct mem_cgroup *memcg, gfp_t gfp)
> @@ -4760,6 +4885,7 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>  	struct mem_cgroup *memcg;
>  	unsigned int size;
>  	int node;
> +	int __maybe_unused i;
>  
>  	size = sizeof(struct mem_cgroup);
>  	size += nr_node_ids * sizeof(struct mem_cgroup_per_node *);
> @@ -4803,6 +4929,9 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
>  #endif
>  #ifdef CONFIG_CGROUP_WRITEBACK
>  	INIT_LIST_HEAD(&memcg->cgwb_list);
> +	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++)
> +		memcg->cgwb_frn[i].done =
> +			__WB_COMPLETION_INIT(&memcg_cgwb_frn_waitq);
>  #endif
>  	idr_replace(&mem_cgroup_idr, memcg, memcg->id.id);
>  	return memcg;
> @@ -4932,7 +5061,12 @@ static void mem_cgroup_css_released(struct cgroup_subsys_state *css)
>  static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
>  {
>  	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
> +	int __maybe_unused i;
>  
> +#ifdef CONFIG_CGROUP_WRITEBACK
> +	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++)
> +		wb_wait_for_completion(&memcg->cgwb_frn[i].done);
> +#endif
>  	if (cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_nosocket)
>  		static_branch_dec(&memcg_sockets_enabled_key);
>  
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 1804f64ff43c..50055d2e4ea8 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1667,6 +1667,8 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
>  		if (unlikely(!writeback_in_progress(wb)))
>  			wb_start_background_writeback(wb);
>  
> +		mem_cgroup_flush_foreign(wb);
> +
>  		/*
>  		 * Calculate global domain's pos_ratio and select the
>  		 * global dtc by default.
> @@ -2427,6 +2429,8 @@ void account_page_dirtied(struct page *page, struct address_space *mapping)
>  		task_io_account_write(PAGE_SIZE);
>  		current->nr_dirtied++;
>  		this_cpu_inc(bdp_ratelimits);
> +
> +		mem_cgroup_track_foreign_dirty(page, wb);
>  	}
>  }
>  
> -- 
> 2.17.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR