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

Oleg Nesterov
2020-09-17T12:12:58Z

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.