Re: [PATCH] mm : bail out from psi memstall after submit_bio in swap_readpage

[Zhaoyang Huang]

On Tue, Sep 7, 2021 at 9:24 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Tue, Sep 07, 2021 at 08:15:30PM +0800, Zhaoyang Huang wrote:
> > On Tue, Sep 7, 2021 at 8:03 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> > >
> > > On 9/7/21 13:59, Huangzhaoyang wrote:
> > > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > > >
> > > > It doesn't make sense to count IO time into psi memstall. Bail out after
> > > > bio submitted.
> > >
> > > Isn't that the point if psi, to observe real stalls, which include IO?
>
> Yes, correct.
>
> > IO stalls could be observed within blk_io_schedule. The time cost of
> > the data from block device to RAM is counted here.
>
> Yes, that is on purpose. The time a thread waits for swap read IO is
> time in which the thread is not productive due to a lack of memory.
>
> For async-submitted IO, this happens in lock_page() called from
> do_swap_page(). If the submitting thread directly waits after the
> submit_bio(), then that should be accounted too.
IMO, memstall counting should be terminated by bio submitted. blk
driver fetching request and the operation on the real device shouldn't
be counted in. It especially doesn't make sense in a virtualization
system like XEN etc, where the blk driver is implemented via
backend-frontend way that introduce  memory irrelevant latency

>
> This patch doesn't make sense to me.