linux-kernel - Re: [PATCH V4 1/2] cgroup/rstat: Helper functions for locking expose trylock

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Date: Thu, 27 Jun 2024 18:22:31 -0400
From: Waiman Long <longman@...hat.com>
To: Jesper Dangaard Brouer <hawk@...nel.org>, tj@...nel.org,
 cgroups@...r.kernel.org, yosryahmed@...gle.com, shakeel.butt@...ux.dev
Cc: hannes@...xchg.org, lizefan.x@...edance.com, kernel-team@...udflare.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 1/2] cgroup/rstat: Helper functions for locking expose
 trylock


On 6/27/24 17:18, Jesper Dangaard Brouer wrote:
> Signed-off-by: Jesper Dangaard Brouer <hawk@...nel.org>
> ---
>   kernel/cgroup/rstat.c |   40 ++++++++++++++++++++++++++++++----------
>   1 file changed, 30 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index fb8b49437573..2a42be3a9bb3 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -279,17 +279,30 @@ __bpf_hook_end();
>    * value -1 is used when obtaining the main lock else this is the CPU
>    * number processed last.
>    */
> -static inline void __cgroup_rstat_lock(struct cgroup *cgrp, int cpu_in_loop)
> +static inline bool __cgroup_rstat_trylock(struct cgroup *cgrp, int cpu_in_loop)
> +{
> +	bool locked;
> +
> +	locked = spin_trylock_irq(&cgroup_rstat_lock);
> +	if (!locked)
> +		trace_cgroup_rstat_lock_contended(cgrp, cpu_in_loop, !locked);
> +
> +	return locked;
> +}
> +
> +static inline void __cgroup_rstat_lock(struct cgroup *cgrp, int cpu_in_loop,
> +				       bool check_contention)
>   	__acquires(&cgroup_rstat_lock)
>   {
> -	bool contended;
> +	bool locked = false;
>   
> -	contended = !spin_trylock_irq(&cgroup_rstat_lock);
> -	if (contended) {
> -		trace_cgroup_rstat_lock_contended(cgrp, cpu_in_loop, contended);
> +	if (check_contention)
> +		locked = __cgroup_rstat_trylock(cgrp, cpu_in_loop);
> +
> +	if (!locked)
>   		spin_lock_irq(&cgroup_rstat_lock);
> -	}
> -	trace_cgroup_rstat_locked(cgrp, cpu_in_loop, contended);
> +
> +	trace_cgroup_rstat_locked(cgrp, cpu_in_loop, !locked);
>   }
>   
>   static inline void __cgroup_rstat_unlock(struct cgroup *cgrp, int cpu_in_loop)
> @@ -328,7 +341,7 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
>   			__cgroup_rstat_unlock(cgrp, cpu);
>   			if (!cond_resched())
>   				cpu_relax();
> -			__cgroup_rstat_lock(cgrp, cpu);
> +			__cgroup_rstat_lock(cgrp, cpu, true);
>   		}
>   	}
>   }
> @@ -348,9 +361,16 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp)
>    */
>   __bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp)
>   {
> +	bool locked;
> +
>   	might_sleep();
>   
> -	__cgroup_rstat_lock(cgrp, -1);
> +	locked = __cgroup_rstat_trylock(cgrp, -1);
> +	if (!locked) {
> +		/* Opportunity to ongoing flush detection */
> +		__cgroup_rstat_lock(cgrp, -1, false);
> +	}
> +
>   	cgroup_rstat_flush_locked(cgrp);
>   	__cgroup_rstat_unlock(cgrp, -1);
>   }
> @@ -368,7 +388,7 @@ void cgroup_rstat_flush_hold(struct cgroup *cgrp)
>   	__acquires(&cgroup_rstat_lock)
>   {
>   	might_sleep();
> -	__cgroup_rstat_lock(cgrp, -1);
> +	__cgroup_rstat_lock(cgrp, -1, true);
>   	cgroup_rstat_flush_locked(cgrp);
>   }
>   
>
>
Will it be cleaner to add a "bool *flushed" output parameter to 
__cgroup_rstat_lock() so that the caller can respond differently whether 
the flushed flag is set or not? In that way, you don't need to expose a 
separate trylock() API. Also your commit log is empty.

Cheers,
Longman