linux-kernel - Re: [PATCH] blk: optimization for classic polling

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Message-ID: <20180221083208.GH25314@hirez.programming.kicks-ass.net>
Date:   Wed, 21 Feb 2018 09:32:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Nitesh Shetty <nj.shetty@...sung.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk, linux-block@...r.kernel.org,
        linux-nvme@...ts.infradead.org, joshi.k@...sung.com
Subject: Re: [PATCH] blk: optimization for classic polling

On Tue, Feb 20, 2018 at 12:37:07PM -1000, Jens Axboe wrote:
> On 2/20/18 3:21 AM, Peter Zijlstra wrote:
> > On Sun, May 30, 2083 at 09:51:06AM +0530, Nitesh Shetty wrote:
> >> This removes the dependency on interrupts to wake up task. Set task
> >> state as TASK_RUNNING, if need_resched() returns true,
> >> while polling for IO completion.
> >> Earlier, polling task used to sleep, relying on interrupt to wake it up.
> >> This made some IO take very long when interrupt-coalescing is enabled in
> >> NVMe.
> > 
> > This is a horrible Changelog.. it does not in fact explain why the patch
> > works or is correct.
> 
> Yeah, that should have been improved.

Being ever more forgetful (I blame the kids) I find Changelogs more and
more important, ymmv ;-)

> > Also, set_current_state(TASK_RUNNING) is dodgy (similarly in
> > __blk_mq_poll), why do you need that memory barrier?
> 
> I pointed that out in the review, and v2 fixed it. v2 is the
> one that got merged.

Right missed that. In fact, possibly the only reason I saw this is that
Nitesh had this computer configured wrong and the email is from the
future and thus the very first entry in my lkml folder.