linux-kernel - Re: [PATCH v2] block: make the io

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Message-ID: <533328c7-4158-d9fd-d8db-0d02eb9c5108@linux.alibaba.com>
Date:   Fri, 27 Dec 2019 00:42:16 +0800
From:   Wen Yang <wenyang@...ux.alibaba.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Joseph Qi <joseph.qi@...ux.alibaba.com>, xlpang@...ux.alibaba.com,
        Mikulas Patocka <mpatocka@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] block: make the io_ticks counter more accurate

On 2019/12/26 11:39 上午, Jens Axboe wrote:
> On 12/25/19 8:10 PM, Wen Yang wrote:
>> Instead of the jiffies, we should update the io_ticks counter
>> with the passed in parameter 'now'.
> 
> I'm still missing some justification for this. What exactly is this
> patch trying to solve or improve? Your commit message says "we should",
> but why?
> 

Hi Jens,

Thank you for your comments.
We observed in the document that:

io_ticks
========

This value counts the number of milliseconds during which the device has
had I/O requests queued.

And the iostat command uses io_ticks count to calculate %util:
https://github.com/sysstat/sysstat/blob/master/rd_stats.c#L372

eg：
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s 
avgrq-sz avgqu-sz   await r_await w_await  svctm  %util

So we need to unify the time windows of these statistics（io_ticks, 
rd_tick, time_in_queue, etc）.
However, the current code uses jiffies to count io_ticks.
Jiffies is different from the passed in parameter 'now',
so these statistics will be inconsistent：

void blk_account_io_done(struct request *req, u64 now)
{
…
     update_io_ticks(part, jiffies);
     part_stat_inc(part, ios[sgrp]);
     part_stat_add(part, nsecs[sgrp], now - req->start_time_ns);
     part_stat_add(part, time_in_queue, nsecs_to_jiffies64(now - 
req->start_time_ns));
…
}

In addition, we also found another issue:
the update_io_tick() function only adds one to io_ticks at a time,
which will result in the calculated %util lower than the real one.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/bio.c#n1713

We will try our best to improve it.
please kindly help with some suggestions.
Thanks.

-- 
Best Regards,
Wen