linux-kernel - Re: [PATCH v2 00/11] md: avoid counter operation conflicts

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Date: Thu, 18 Apr 2024 17:58:11 +0800
From: Xiao Ni <xni@...hat.com>
To: tada keisuke <keisuke1.tada@...xia.com>
Cc: "song@...nel.org" <song@...nel.org>, "yukuai3@...wei.com" <yukuai3@...wei.com>, 
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 00/11] md: avoid counter operation conflicts

On Thu, Apr 18, 2024 at 2:21 PM tada keisuke <keisuke1.tada@...xia.com> wrote:
>
> Changes in v2:
>  - Add message of performance in cover.
>  - Fix a problem of error code initialization in patch 6.
>  - Fix a problem of sleeping during rcu_read_lock() in patch 9.
>  - Change base-commit from md-6.9 to md-6.10
>
> Currently, active_aligned_reads and nr_pending used as counters are atomic types.
> Therefore, when inc/dec in a multi-core results in conflicts and READ I/O becomes slow.
> To improve performance, use "percpu_ref" counters that can avoid conflicts and maintain consistency.
>
> Switch modes of percpu_ref to achieve both consistency and conflict avoidance.
> During normal operations such as inc/dec, it operates as percpu mode.
> When consistency is required, it operates as atomic mode.
> The operations that require consistency are as follows:
>  - Zero check for the counter
>  - All operations in RAID 1/10
>
> Patches 1, 3, 6 change active_aligned_reads, and patches 2, 4, 5, 7 to 11 change nr_pending.
> nr_pending temporarily switch from percpu mode to atomic mode in patch 7.
> This is to reduce the amount of changes from patches 8 to 10.
> Finally, nr_pending switch from atomic mode to percpu mode in patch 11.
>
> We applied the patch to base-commit and used fio to compare IOPS.
> CPU: AMD EPYC 7313P (3.0GHz, 16cores)
> DISK: ramdisk x 3 (modprobe brd rd_nr=3)
> RAID: level 5
> fio config: bs=4k, rw=randread, iodepth=128, numjobs=16
>
> without patch: 3.64 MIOPS
> with patch   : 3.84 MIOPS

Hi Tada

Thanks for the patch set. Have you done tests with nvme/ssd or hdd?
It's better to see the results with real disks.

Best Regards
Xiao
>
> Keisuke TADA (11):
>   add infra for active_aligned_reads changes
>   add infra for nr_pending changes
>   workaround for inconsistency of config state in takeover
>   minimize execution of zero check for nr_pending
>   match the type of variables to percpu_ref
>   avoid conflicts in active_aligned_reads operations
>   change the type of nr_pending from atomic_t to percpu_ref
>   add atomic mode switching in RAID 1/10
>   add atomic mode switching when removing disk
>   add atomic mode switching when I/O completion
>   avoid conflicts in nr_pending operations
>
>  drivers/md/md-bitmap.c   |  2 +-
>  drivers/md/md.c          | 48 ++++++++++++++++++---
>  drivers/md/md.h          | 62 +++++++++++++++++++++++----
>  drivers/md/raid1.c       | 37 +++++++++++------
>  drivers/md/raid10.c      | 60 ++++++++++++++++-----------
>  drivers/md/raid5-cache.c |  4 +-
>  drivers/md/raid5.c       | 90 +++++++++++++++++++++++++++-------------
>  drivers/md/raid5.h       | 17 +++++++-
>  8 files changed, 238 insertions(+), 82 deletions(-)
>
>
> base-commit: 9d1110f99c253ccef82e480bfe9f38a12eb797a7
> --
> 2.34.1
>
>
>