linux-kernel - Re: [RFC PATCH] mm: count zram read/write into PSI_IO

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Date:   Wed, 8 Dec 2021 11:38:37 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Jens Axboe <axboe@...nel.dk>, Minchan Kim <minchan@...nel.org>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm: count zram read/write into PSI_IO_WAIT

On Fri, Dec 03, 2021 at 05:16:47PM +0800, Zhaoyang Huang wrote:
> On Fri, Dec 3, 2021 at 12:28 AM Johannes Weiner <hannes@...xchg.org> wrote:
> >
> > On Wed, Dec 01, 2021 at 07:12:30PM +0800, Zhaoyang Huang wrote:
> > > There is no chance for zram reading/writing to be counted in
> > > PSI_IO_WAIT so far as zram will deal with the request just in current
> > > context without invoking submit_bio and io_schedule.
> >
> > Hm, but you're also not waiting for a real io device - during which
> > the CPU could be doing something else e.g. You're waiting for
> > decompression. The thread also isn't in D-state during that time. What
> > scenario would benefit from this accounting? How is IO pressure from
> > comp/decomp paths actionable to you?
> No. Block device related D-state will be counted in via
> psi_dequeue(io_wait). What I am proposing here is do NOT ignore the
> influence on non-productive time by huge numbers of in-context swap
> in/out (zram like). This can help to make IO pressure more accurate
> and coordinate with the number of PSWPIN/OUT. It is like counting the
> IO time within filemap_fault->wait_on_page_bit_common into
> psi_mem_stall, which introduces memory pressure high by IO.

It's not ignored, it shows up as memory pressure. Because those delays
occur due to a lack of memory.

On the other hand, having a faster IO device would make no difference
to the time spent on compression and decompression. Counting this time
as IO pressure makes no sense to me.

I'm against merging this patch.