linux-kernel - Re: [PATCH] bcache: add cond_resched() in __bch_cache

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Message-ID: <20190307154421.GA8829@suse.cz>
Date:   Thu, 7 Mar 2019 16:44:21 +0100
From:   Vojtech Pavlik <vojtech@...e.com>
To:     Coly Li <colyli@...e.de>
Cc:     Shile Zhang <shile.zhang@...ux.alibaba.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bcache: add cond_resched() in __bch_cache_cmp()

On Thu, Mar 07, 2019 at 11:36:18PM +0800, Coly Li wrote:
> On 2019/3/7 11:06 下午, Shile Zhang wrote:
> > 
> > On 2019/3/7 18:34, Coly Li wrote:
> >> On 2019/3/7 1:15 下午, shile.zhang@...ux.alibaba.com wrote:
> >>> From: Shile Zhang <shile.zhang@...ux.alibaba.com>
> >>>
> >>> Read /sys/fs/bcache/<uuid>/cacheN/priority_stats can take very long
> >>> time with huge cache after long run.
> >>>
> >>> Signed-off-by: Shile Zhang <shile.zhang@...ux.alibaba.com>
> >> Hi Shile,
> >>
> >> Do you test your change ? It will be helpful with more performance data
> >> (what problem that you improved).
> > 
> > In case of 960GB SSD cache device, once read of the 'priority_stats'
> > costs about 600ms in our test environment.
> > 
> 
> After the fix, how much time it takes ?
> 
> 
> > The perf tool shown that near 50% CPU time consumed by 'sort()', this
> > means once sort will hold the CPU near 300ms.
> > 
> > In our case, the statistics collector reads the 'priority_stats'
> > periodically, it will trigger the schedule latency jitters of the
> > 
> > task which shared same CPU core.
> > 
> 
> Hmm, it seems you just make the sort slower, and nothing more changes.
> Am I right ?

Well, it has to make the sort slower, but it'll also avoid hogging the
CPU (on a non-preemptible kernel), avoiding a potential soft lockup
warning and allowing other tasks to run.

-- 
Vojtech Pavlik
VP SUSE Labs