linux-kernel - Re: [GIT PULL] slab updates for 6.11

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Message-ID: <0295538a-4b79-42bf-b0e1-5a905749de1e@suse.cz>
Date: Thu, 26 Sep 2024 18:35:27 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Uladzislau Rezki <urezki@...il.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Cc: David Rientjes <rientjes@...gle.com>, Christoph Lameter <cl@...ux.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 LKML <linux-kernel@...r.kernel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, Christian Brauner <brauner@...nel.org>,
 RCU <rcu@...r.kernel.org>, Shakeel Butt <shakeel.butt@...ux.dev>
Subject: Re: [GIT PULL] slab updates for 6.11

On 9/18/24 16:40, Uladzislau Rezki wrote:
>>
> Thank you for valuable feedback! Indeed it is hard to follow, even
> though it works correctly.
> I will add the comment and also break the loop on first queuing as you
> suggested!
> 
> It does not make sense to loop further because following iterations
> are never successful
> thus never overwrite "queued" variable(it never reaches the
> queue_rcu_work() call).
> 
> <snip>
>          bool queued = false;
>          ...
>          for (i = 0; i < KFREE_N_BATCHES; i++) {
>                 if (need_offload_krc(krcp)) {
>                          queued = queue_rcu_work(system_wq, &krwp->rcu_work);
>          ...
>          return queued;
> <snip>
> 
> if we queued, "if(need_offload_krc())" condition is never true anymore.
> 
> Below refactoring makes it clear. I will send the patch to address it.

Looks good, AFAICT. Can you send the full patch then? Thanks.

> <snip>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a60616e69b66..b1f883fcd918 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3607,11 +3607,12 @@ kvfree_rcu_queue_batch(struct kfree_rcu_cpu *krcp)
>                         }
> 
>                         // One work is per one batch, so there are three
> -                       // "free channels", the batch can handle. It can
> -                       // be that the work is in the pending state when
> -                       // channels have been detached following by each
> -                       // other.
> +                       // "free channels", the batch can handle. Break
> +                       // the loop since it is done with this CPU thus
> +                       // queuing an RCU work is _always_ success here.
>                         queued = queue_rcu_work(system_unbound_wq,
> &krwp->rcu_work);
> +                       WARN_ON_ONCE(!queued);
> +                       break;
>                 }
>         }
> <snip>
> 
> Thanks!
>