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Date:   Wed, 30 Mar 2022 08:34:08 +0000
From:   CGEL <cgel.zte@gmail.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     axboe@kernel.dk, viro@zeniv.linux.org.uk,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, akpm@linux-foundation.org,
        Yang Yang <yang.yang29@zte.com.cn>,
        Ran Xiaokai <ran.xiaokai@zte.com.cn>
Subject: Re: [PATCH] block/psi: make PSI annotations of submit_bio only work
 for file pages
References: <20220316063927.2128383-1-yang.yang29@zte.com.cn>
 <YjiMsGoXoDU+FwsS@cmpxchg.org>
 <623938d1.1c69fb81.52716.030f@mx.google.com>
 <YjnO3p6vvAjeMCFC@cmpxchg.org>
 <20220323061058.GA2343452@cgel.zte@gmail.com>
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On Wed, Mar 23, 2022 at 06:11:03AM +0000, CGEL wrote:
> On Tue, Mar 22, 2022 at 09:27:58AM -0400, Johannes Weiner wrote:
> > On Tue, Mar 22, 2022 at 02:47:42AM +0000, CGEL wrote:
> > > On Mon, Mar 21, 2022 at 10:33:20AM -0400, Johannes Weiner wrote:
> > > > On Wed, Mar 16, 2022 at 06:39:28AM +0000, cgel.zte@gmail.com wrote:
> > > > > From: Yang Yang <yang.yang29@zte.com.cn>
> > > > > 
> > > > > psi tracks the time spent on submitting the IO of refaulting file pages
> > > > > and anonymous pages[1]. But after we tracks refaulting anonymous pages
> > > > > in swap_readpage[2][3], there is no need to track refaulting anonymous
> > > > > pages in submit_bio.
> > > > > 
> > > > > So this patch can reduce redundant calling of psi_memstall_enter. And
> > > > > make it easier to track refaulting file pages and anonymous pages
> > > > > separately.
> > > > 
> > > > I don't think this is an improvement.
> > > > 
> > > > psi_memstall_enter() will check current->in_memstall once, detect the
> > > > nested call, and bail. Your patch checks PageSwapBacked for every page
> > > > being added. It's more branches for less robust code.
> > > 
> > > We are also working for a new patch to classify different reasons cause
> > > psi_memstall_enter(): reclaim, thrashing, compact, etc. This will help
> > > user to tuning sysctl, for example, if user see high compact delay, he
> > > may try do adjust THP sysctl to reduce the compact delay.
> > > 
> > > To support that, we should distinguish what's the reason cause psi in
> > > submit_io(), this patch does the job.
> > 
> > Please submit these patches together then. On its own, this patch
> > isn't desirable.
> I think this patch has it's independent value, I try to make a better
> explain.
> 
> 1) This patch doesn't work it worse or even better
> After this patch, swap workingset handle is simpler, file workingset
> handle just has one more check, as below.
> Before this patch handling swap workingset:
> 	a) in swap_readpage() call psi_memstall_enter() ->
> 	b) in __bio_add_page() test if should call bio_set_flag(), true ->
> 	c) in __bio_add_page() call bio_set_flag()
> 	d) in submit_bio() test if should call psi_memstall_enter(), true ->
> 	e) call psi_memstall_enter, detect the nested call, and bail.
> 	f) call bio_clear_flag if needed.
> Before this patch handling file page workingset:
> 	a) in __bio_add_page() test if should call bio_set_flag(), true ->
> 	...
> 	b) call bio_clear_flag if needed.
> After this patch handling swap workingset:
> 	a) in swap_readpage() call psi_memstall_enter() ->
> 	b) in __bio_add_page() test if should call bio_set_flag(), one more check, false and return.
> 	c) in submit_bio() test if should call psi_memstall_enter(), false and return.
> After this patch handling file pages workingset:
> 	a) in __bio_add_page() test if should call bio_set_flag(), one more check, true ->
> 	...
> 	b) call bio_clear_flag if needed.
> 
> 2) This patch help tools like kprobe to trace different workingset
> After this patch we know workingset in submit_io() is only cause by file pages.
> 
> 3) This patch will help code evolution
> Such as psi classify, getdelays supports counting file pages workingset submit.

Looking forward to your review, thanks!