linux-kernel - Re: [PATCH 1/1] psi: remove 500ms min window size limitation for triggers

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Date:   Thu, 2 Mar 2023 14:58:25 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     tj@...nel.org, hannes@...xchg.org, lizefan.x@...edance.com,
        peterz@...radead.org, johunt@...mai.com, keescook@...omium.org,
        quic_sudaraja@...cinc.com, cgroups@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] psi: remove 500ms min window size limitation for
 triggers

On Wed 01-03-23 11:34:03, Suren Baghdasaryan wrote:
> Current 500ms min window size for psi triggers limits polling interval
> to 50ms to prevent polling threads from using too much cpu bandwidth by
> polling too frequently. However the number of cgroups with triggers is
> unlimited, so this protection can be defeated by creating multiple
> cgroups with psi triggers (triggers in each cgroup are served by a single
> "psimon" kernel thread).
> Instead of limiting min polling period, which also limits the latency of
> psi events, it's better to limit psi trigger creation to authorized users
> only, like we do for system-wide psi triggers (/proc/pressure/* files can
> be written only by processes with CAP_SYS_RESOURCE capability). This also
> makes access rules for cgroup psi files consistent with system-wide ones.
> Add a CAP_SYS_RESOURCE capability check for cgroup psi file writers and
> remove the psi window min size limitation.
> 
> Suggested-by: Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>
> Link: https://lore.kernel.org/all/cover.1676067791.git.quic_sudaraja@quicinc.com/
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>

Acked-by: Michal Hocko <mhocko@...e.com>

with this to fix
[...]
> @@ -1278,8 +1277,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
>  	if (state >= PSI_NONIDLE)
>  		return ERR_PTR(-EINVAL);
>  
> -	if (window_us < WINDOW_MIN_US ||
> -		window_us > WINDOW_MAX_US)
> +	if (window_us <= 0 || window_us > WINDOW_MAX_US)
>  		return ERR_PTR(-EINVAL);

window_us is u32 sp the check for <= 0 doesn't make any sense.

>  
>  	/* Check threshold */
> -- 
> 2.40.0.rc0.216.gc4246ad0f0-goog

-- 
Michal Hocko
SUSE Labs