linux-kernel - Re: [PATCH v3 5/5] psi: introduce psi monitor

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Message-ID: <20190129104431.GJ28467@hirez.programming.kicks-ass.net>
Date:   Tue, 29 Jan 2019 11:44:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     gregkh@...uxfoundation.org, tj@...nel.org, lizefan@...wei.com,
        hannes@...xchg.org, axboe@...nel.dk, dennis@...nel.org,
        dennisszhou@...il.com, mingo@...hat.com, akpm@...ux-foundation.org,
        corbet@....net, cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor

On Thu, Jan 24, 2019 at 01:15:18PM -0800, Suren Baghdasaryan wrote:
>  static void psi_update_work(struct work_struct *work)
>  {
>  	struct delayed_work *dwork;
>  	struct psi_group *group;
> +	bool first_pass = true;
> +	u64 next_update;
> +	u32 change_mask;
> +	int polling;
>  	bool nonidle;
> +	u64 now;
>  
>  	dwork = to_delayed_work(work);
>  	group = container_of(dwork, struct psi_group, clock_work);
>  
> +	now = sched_clock();
> +
> +	mutex_lock(&group->update_lock);

actually acquiring a mutex can take a fairly long while; would it not
make more sense to take the @now timestanp _after_ it, instead of
before?