linux-kernel - Re: [PATCH v3 2/2] io_uring/io-wq: inherit cpuset of cgroup in io worker

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Message-ID: <1589cf94-6d27-4575-bcea-e36c3667b260@redhat.com>
Date: Tue, 10 Sep 2024 13:42:30 -0400
From: Waiman Long <longman@...hat.com>
To: Felix Moessbauer <felix.moessbauer@...mens.com>, axboe@...nel.dk
Cc: asml.silence@...il.com, linux-kernel@...r.kernel.org,
 io-uring@...r.kernel.org, cgroups@...r.kernel.org, dqminh@...udflare.com,
 adriaan.schmidt@...mens.com, florian.bezdeka@...mens.com
Subject: Re: [PATCH v3 2/2] io_uring/io-wq: inherit cpuset of cgroup in io
 worker


On 9/10/24 13:11, Felix Moessbauer wrote:
> The io worker threads are userland threads that just never exit to the
> userland. By that, they are also assigned to a cgroup (the group of the
> creating task).

The io-wq task is not actually assigned to a cgroup. To belong to a 
cgroup, its pid has to be present to the cgroup.procs of the 
corresponding cgroup, which is not the case here. My understanding is 
that you are just restricting the CPU affinity to follow the cpuset of 
the corresponding user task that creates it. The CPU affinity (cpumask) 
is just one of the many resources controlled by a cgroup. That probably 
needs to be clarified.

Besides cpumask, the cpuset controller also controls the node mask of 
the memory nodes allowed.

Cheers,
Longman

>
> When creating a new io worker, this worker should inherit the cpuset
> of the cgroup.
>
> Fixes: da64d6db3bd3 ("io_uring: One wqe per wq")
> Signed-off-by: Felix Moessbauer <felix.moessbauer@...mens.com>
> ---
>   io_uring/io-wq.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/io_uring/io-wq.c b/io_uring/io-wq.c
> index c7055a8895d7..a38f36b68060 100644
> --- a/io_uring/io-wq.c
> +++ b/io_uring/io-wq.c
> @@ -1168,7 +1168,7 @@ struct io_wq *io_wq_create(unsigned bounded, struct io_wq_data *data)
>   
>   	if (!alloc_cpumask_var(&wq->cpu_mask, GFP_KERNEL))
>   		goto err;
> -	cpumask_copy(wq->cpu_mask, cpu_possible_mask);
> +	cpuset_cpus_allowed(data->task, wq->cpu_mask);
>   	wq->acct[IO_WQ_ACCT_BOUND].max_workers = bounded;
>   	wq->acct[IO_WQ_ACCT_UNBOUND].max_workers =
>   				task_rlimit(current, RLIMIT_NPROC);