linux-kernel - Re: [PATCH 3/3] mm: add vmstat statistics for madvise

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Message-ID: <Y8gr5haAMtQJJSR6@dhcp22.suse.cz>
Date:   Wed, 18 Jan 2023 18:27:02 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        SeongJae Park <sj@...nel.org>
Subject: Re: [PATCH 3/3] mm: add vmstat statistics for madvise_[cold|pageout]

On Wed 18-01-23 09:15:34, Minchan Kim wrote:
> On Wed, Jan 18, 2023 at 10:11:46AM +0100, Michal Hocko wrote:
> > On Tue 17-01-23 15:16:32, Minchan Kim wrote:
> > > madvise LRU manipulation APIs need to scan address ranges to find
> > > present pages at page table and provides advice hints for them.
> > > 
> > > Likewise pg[scan/steal] count on vmstat, madvise_pg[scanned/hinted]
> > > shows the proactive reclaim efficiency so this patch addes those
> > > two statistics in vmstat.
> > 
> > Please describe the usecase for those new counters.
> 
> I wanted to know the proactive reclaim efficieny using MADV_COLD/MDDV_PAGEOUT.
> Userspace has several policy which when/which vmas need to be hinted by the call
> and they are evolving. I needed to know how effectively their policy works since
> the vma ranges are huge(i.e., nr_hinted/nr_scanned).

I can see how that can be an interesting information but is there
anything actionable about that beyond debugging purposes? In other words
isn't this something that could be done by tracing instead?

Also how are you going to identify specific madvise calls when they can
interleave arbitrarily?

-- 
Michal Hocko
SUSE Labs