linux-kernel - Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read

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Date:   Thu, 17 Sep 2020 14:01:33 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Boaz Harrosh <boaz@...xistor.com>
Cc:     Hou Tao <houtao1@...wei.com>, peterz@...radead.org,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for
 read_count

On 09/17, Boaz Harrosh wrote:
>
> On 16/09/2020 15:32, Hou Tao wrote:
> <>
> >However the performance degradation is huge under aarch64 (4 sockets, 24 core per sockets): nearly 60% lost.
> >
> >v4.19.111
> >no writer, reader cn                               | 24        | 48        | 72        | 96
> >the rate of down_read/up_read per second           | 166129572 | 166064100 | 165963448 | 165203565
> >the rate of down_read/up_read per second (patched) |  63863506 |  63842132 |  63757267 |  63514920
> >
>
> I believe perhaps Peter Z's suggestion of an additional
> percpu_down_read_irqsafe() API and let only those in IRQ users pay the
> penalty.
>
> Peter Z wrote:
> >My leading alternative was adding: percpu_down_read_irqsafe() /
> >percpu_up_read_irqsafe(), which use local_irq_save() instead of
> >preempt_disable().

This means that __sb_start/end_write() and probably more users in fs/super.c
will have to use this API, not good.

IIUC, file_end_write() was never IRQ safe (at least if !CONFIG_SMP), even
before 8129ed2964 ("change sb_writers to use percpu_rw_semaphore"), but this
doesn't matter...

Perhaps we can change aio.c, io_uring.c and fs/overlayfs/file.c to avoid
file_end_write() in IRQ context, but I am not sure it's worth the trouble.

Oleg.