Re: [PATCH 16/16] rcu/tree: Use delayed work instead of hrtimer to refill the cache

[Paul E. McKenney]

On Thu, Oct 29, 2020 at 05:50:19PM +0100, Uladzislau Rezki (Sony) wrote:
> A CONFIG_PREEMPT_COUNT is unconditionally enabled, thus a page
> can be obtained directly from a kvfree_rcu() path. To distinguish
> that and take a decision the preemptable() macro is used when it
> is save to enter allocator.
> 
> It means that refilling a cache is not important from timing point
> of view. Switch to a delayed work, so the actual work is queued from
> the timer interrupt with 1 jiffy delay. An immediate placing a task
> on a current CPU can lead to rq->lock double lock. That is why a
> delayed method is in place.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>

Thank you, Uladzislau!

I applied this on top of v5.10-rc1 and got the following from the
single-CPU builds:

  SYNC    include/config/auto.conf.cmd
  DESCEND  objtool
  CC      kernel/bounds.s
  CALL    scripts/atomic/check-atomics.sh
  UPD     include/generated/bounds.h
  CC      arch/x86/kernel/asm-offsets.s
In file included from ./include/asm-generic/atomic-instrumented.h:20:0,
                 from ./include/linux/atomic.h:82,
                 from ./include/linux/crypto.h:15,
                 from arch/x86/kernel/asm-offsets.c:9:
./include/linux/pagemap.h: In function ‘__page_cache_add_speculative’:
./include/linux/build_bug.h:30:34: error: called object is not a function or function pointer
 #define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
                                 ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./include/linux/mmdebug.h:45:25: note: in expansion of macro ‘BUILD_BUG_ON_INVALID’
 #define VM_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
                         ^~~~~~~~~~~~~~~~~~~~
./include/linux/pagemap.h:207:2: note: in expansion of macro ‘VM_BUG_ON’
  VM_BUG_ON(preemptible())
  ^~~~~~~~~
scripts/Makefile.build:117: recipe for target 'arch/x86/kernel/asm-offsets.s' failed
make[1]: *** [arch/x86/kernel/asm-offsets.s] Error 1
Makefile:1199: recipe for target 'prepare0' failed
make: *** [prepare0] Error 2

I vaguely recall something like this showing up in the previous series
and that we did something or another to address it.  Could you please
check against the old series at -rcu branch dev.2020.10.22a?  (I verified
that the old series does run correctly in the single-CPU scenarios.)

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 26 +++++---------------------
>  1 file changed, 5 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 3f9b016a44dc..9ea55f800b4b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3086,7 +3086,6 @@ struct kfree_rcu_cpu_work {
>   *	per-cpu lock.
>   * @page_cache_work: A work to refill the cache when it is empty
>   * @work_in_progress: Indicates that page_cache_work is running
> - * @hrtimer: A hrtimer for scheduling a page_cache_work
>   * @nr_bkv_objs: number of allocated objects at @bkvcache.
>   *
>   * This is a per-CPU structure.  The reason that it is not included in
> @@ -3104,9 +3103,8 @@ struct kfree_rcu_cpu {
>  	bool initialized;
>  	int count;
>  
> -	struct work_struct page_cache_work;
> +	struct delayed_work page_cache_work;
>  	atomic_t work_in_progress;
> -	struct hrtimer hrtimer;
>  
>  	struct llist_head bkvcache;
>  	int nr_bkv_objs;
> @@ -3355,22 +3353,12 @@ static void kfree_rcu_monitor(struct work_struct *work)
>  		raw_spin_unlock_irqrestore(&krcp->lock, flags);
>  }
>  
> -static enum hrtimer_restart
> -schedule_page_work_fn(struct hrtimer *t)
> -{
> -	struct kfree_rcu_cpu *krcp =
> -		container_of(t, struct kfree_rcu_cpu, hrtimer);
> -
> -	queue_work(system_highpri_wq, &krcp->page_cache_work);
> -	return HRTIMER_NORESTART;
> -}
> -
>  static void fill_page_cache_func(struct work_struct *work)
>  {
>  	struct kvfree_rcu_bulk_data *bnode;
>  	struct kfree_rcu_cpu *krcp =
>  		container_of(work, struct kfree_rcu_cpu,
> -			page_cache_work);
> +			page_cache_work.work);
>  	unsigned long flags;
>  	bool pushed;
>  	int i;
> @@ -3398,12 +3386,8 @@ static void
>  run_page_cache_worker(struct kfree_rcu_cpu *krcp)
>  {
>  	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
> -			!atomic_xchg(&krcp->work_in_progress, 1)) {
> -		hrtimer_init(&krcp->hrtimer, CLOCK_MONOTONIC,
> -			HRTIMER_MODE_REL);
> -		krcp->hrtimer.function = schedule_page_work_fn;
> -		hrtimer_start(&krcp->hrtimer, 0, HRTIMER_MODE_REL);
> -	}
> +			!atomic_xchg(&krcp->work_in_progress, 1))
> +		schedule_delayed_work(&krcp->page_cache_work, 1);
>  }
>  
>  // Record ptr in a page managed by krcp, with the pre-krc_this_cpu_lock()
> @@ -4503,7 +4487,7 @@ static void __init kfree_rcu_batch_init(void)
>  		}
>  
>  		INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
> -		INIT_WORK(&krcp->page_cache_work, fill_page_cache_func);
> +		INIT_DELAYED_WORK(&krcp->page_cache_work, fill_page_cache_func);
>  		krcp->initialized = true;
>  	}
>  	if (register_shrinker(&kfree_rcu_shrinker))
> -- 
> 2.20.1
>