linux-kernel - Re: [PATCH v2 0/1] scale loop device lock

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Date:   Tue, 19 Jan 2021 12:12:19 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>, axboe@...nel.dk,
        linux-block@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/1] scale loop device lock

On Thu, Jul 23, 2020 at 5:17 PM Pavel Tatashin
<pasha.tatashin@...een.com> wrote:

It has been half a year, and no activity on this patch. Can it be applied?

Thanks,
Pasha

>
> Changelog
> v2: Addressed Tyler Hicks comments
>         - added mutex_destroy()
>         - comment in lo_open()
>         - added lock around lo_disk in
>
>
> ===
>
> In our environment we are using systemd portable containers in
> squashfs formats, convert them into loop device, and mount.
>
> NAME                      MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
> loop5                       7:5    0  76.4M  0 loop
> `-BaseImageM1908          252:3    0  76.4M  1 crypt /BaseImageM1908
> loop6                       7:6    0    20K  0 loop
> `-test_launchperf20       252:17   0   1.3M  1 crypt /app/test_launchperf20
> loop7                       7:7    0    20K  0 loop
> `-test_launchperf18       252:4    0   1.5M  1 crypt /app/test_launchperf18
> loop8                       7:8    0     8K  0 loop
> `-test_launchperf8        252:25   0    28K  1 crypt app/test_launchperf8
> loop9                       7:9    0   376K  0 loop
> `-test_launchperf14       252:29   0  45.7M  1 crypt /app/test_launchperf14
> loop10                      7:10   0    16K  0 loop
> `-test_launchperf4        252:11   0   968K  1 crypt app/test_launchperf4
> loop11                      7:11   0   1.2M  0 loop
> `-test_launchperf17       252:26   0 150.4M  1 crypt /app/test_launchperf17
> loop12                      7:12   0    36K  0 loop
> `-test_launchperf19       252:13   0   3.3M  1 crypt /app/test_launchperf19
> loop13                      7:13   0     8K  0 loop
> ...
>
> We have over 50 loop devices which are mounted  during boot.
>
> We observed contentions around loop_ctl_mutex.
>
> The sample contentions stacks:
>
> Contention 1:
> __blkdev_get()
>    bdev->bd_disk->fops->open()
>       lo_open()
>          mutex_lock_killable(&loop_ctl_mutex); <- contention
>
> Contention 2:
> __blkdev_put()
>    disk->fops->release()
>       lo_release()
>          mutex_lock(&loop_ctl_mutex); <- contention
>
> With total time waiting for loop_ctl_mutex ~18.8s during boot (across 8
> CPUs) on our machine (69 loop devices): 2.35s per CPU.
>
> Scaling this lock eliminates this contention entirely, and improves the boot
> performance by 2s on our machine.
>
> Pavel Tatashin (1):
>   loop: scale loop device by introducing per device lock
>
>  drivers/block/loop.c | 99 ++++++++++++++++++++++++++------------------
>  drivers/block/loop.h |  1 +
>  2 files changed, 59 insertions(+), 41 deletions(-)
>
> --
> 2.25.1
>