linux-kernel - Re: [PATCH 4/8] Btrfs: stop using btrfs_schedule

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Message-ID: <20190614134501.cjnrsb5n6jtteqex@MacBook-Pro-91.local>
Date:   Fri, 14 Jun 2019 09:45:02 -0400
From:   Josef Bacik <josef@...icpanda.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     dsterba@...e.com, clm@...com, josef@...icpanda.com,
        axboe@...nel.dk, jack@...e.cz, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 4/8] Btrfs: stop using btrfs_schedule_bio()

On Thu, Jun 13, 2019 at 05:33:46PM -0700, Tejun Heo wrote:
> From: Chris Mason <clm@...com>
> 
> btrfs_schedule_bio() hands IO off to a helper thread to do the actual
> submit_bio() call.  This has been used to make sure async crc and
> compression helpers don't get stuck on IO submission.  To maintain good
> performance, over time the IO submission threads duplicated some IO
> scheduler characteristics such as high and low priority IOs and they
> also made some ugly assumptions about request allocation batch sizes.
> 
> All of this cost at least one extra context switch during IO submission,
> and doesn't fit well with the modern blkmq IO stack.  So, this commit stops
> using btrfs_schedule_bio().  We may need to adjust the number of async
> helper threads for crcs and compression, but long term it's a better
> path.
> 
> Signed-off-by: Chris Mason <clm@...com>

Reviewed-by: Josef Bacik <josef@...icpanda.com>

Thanks,

Josef