Re: [PATCH] psi: Treat ksm swapping in copy as memstall

[Hugh Dickins <hughd@google.com>]

On Tue, 8 Feb 2022, CGEL wrote:
> On Mon, Feb 07, 2022 at 07:22:22PM -0800, Hugh Dickins wrote:
> > On Fri, 28 Jan 2022, CGEL wrote:
> > > On Thu, Jan 27, 2022 at 08:29:08PM -0500, Johannes Weiner wrote:
> > > > On Fri, Jan 21, 2022 at 09:51:08AM +0000, CGEL wrote:
> > > > >  Wed, Jan 19, 2022 at 07:58:23AM -0500, Johannes Weiner wrote:
> > > > > > On Wed, Jan 19, 2022 at 06:13:54AM +0000, CGEL wrote:
> > > > > > > I did a test, when we use zram, it takes longer time for ksm copying than
> > > > > > > swap_readpage(). Ksm copying average takes 147263ns, swap_readpage()
> > > > > > > average takes 55639ns. So I think this patch is reasonable.
> > > > > > 
> > > > > > Ok, that sounds reasonable to me as well. Please add the
> > > > > > PageWorkingset() check and resubmit the patch. Thanks!
> > > > > I am a litte confused about adding PageWorkingset(), since I
> > > > > think ksm_might_need_to_copy() memstall is like swap_readpage()
> > > > > memstall and swap_readpage() doesn't add PageWorkingset().
> > > > 
> > > > That's actually a bug! It should do that.
> > > I recently found that too. Please CC to me your new patch, thanks!
> > > And I will send V2 of this patch "psi: Treat ksm swapping in copy
> > > as memstall" with PageWorkingset().
> > 
> > I'm entirely PSI-ignorant, and reluctant to disagree with Johannes,
> > but I don't see how your patch to ksm_might_need_to_copy() could be
> > correct - maybe the "swapping" in your subject is confusing.
> > 
> > There is no PSI enter and exit around the page allocation and copying
> > in wp_page_copy(), so why in the analogous ksm_might_need_to_copy()?
> >
> I think it's two questions, first why PSI didn't treat wp_page_copy() as
> memstall, second why should PSI treat ksm_might_need_to_copy() as memstall.
> 
> The first question is unrelated with this patch. I think the reason is PSI
> focous on memory contending(see Documentation/accounting/psi.rst), and
> wp_page_copy() is not caused by memory contending. Actually wp_page_copy()
> will still be called if memory is not contending.

Agreed.

> 
> For the second question, ksm_might_need_to_copy() is called only becaused
> of swapping, and swap is caused by memory contending, so PSI better treat
> it as memstall.

But there I'm not at all convinced.

 * psi_memstall_enter - mark the beginning of a memory stall section
 * @flags: flags to handle nested sections
 *
 * Marks the calling task as being stalled due to a lack of memory,
 * such as waiting for a refault or performing reclaim.

psi_memstall_enter() will have been called if do_swap_page() had to
read back from swap or wait on page lock; and psi_memstall_enter()
will be called if ksm_might_need_to_copy()'s alloc_page_vma() or
mem_cgroup_charge() goes into page reclaim.  Being stalled due to
a lack of memory is fully covered there.

Your argument is that copy_user_highpage() is a significant overhead
(but not a memstall), and it might have been avoided if there was
no KSM or no swapping - though doing the copy there often(?) saves
doing the wp_page_copy() when do_swap_page() goes on to do_wp_page()
(and that one you're not asking to count).

I can understand you wanting to keep track of page copying overhead;
and I've grown uneasy recently about the way CONFIG_KSM=y can add a
ksm_might_need_to_copy() overhead for pages which never went near KSM;
but I still don't see how psi_memstall is appropriate here.

Hugh