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

[Johannes Weiner]

On Wed, Sep 08, 2021 at 11:35:40AM +0800, Zhaoyang Huang wrote:
> 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

Yes but the entire IO operation and all the associated latency only
happens due to a shortage of memory in the first place. The thread is
incurring these delays due to a lack of memory.

What is a memstall if not the latencies and wait times incurred in the
process of reloading pages that were evicted prematurely?