linux-kernel - Re: [PATCH 1/3] mm: return the number of pages successfully paged out

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Date:   Thu, 19 Jan 2023 13:15:36 -0800
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
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 1/3] mm: return the number of pages successfully paged out

On Thu, Jan 19, 2023 at 10:07:23AM +0100, Michal Hocko wrote:
> On Wed 18-01-23 14:27:23, Minchan Kim wrote:
> [...]
> > Let me know if you have other concern or suggestion.
> 
> I would propose to use a tracepoint to track this on the madvise side.
> This way you can both track a per-process effectivity as well a madvise
> originator effectivity (if the policy is implemented by a global monitor
> then it won't get interfering activity by other users of this
> interface). Global counters cannot do neither of that.

I don't think the tracepoint is right approach for the purpose.

I understand we could get the same result using tracepoint using bpf or
something so whenever event happens, a daemon get the result and
accumlate the number so totally same result with global stat. Yeah,
technically it's doable. With the claim, there is nothing we can do
with tracpoint. Checks existing vmstat fields, why do we have them
into vmstat instead of tracepoint? TP is much easiler/fleixible but
with vmstat, we can get the ballpark under fleet easier to sense
what's going on simply, and once we found something weird, we could
turn on the trace to know the detail and TP would work for it.

With process control using process_madvise in centralized controlled
system, I think those two stats are really worth along with other
memory reclaim statistics to be captured for memory health. If we
have needs per-process level tracking(Actually, not for our case),
we could add the tracepoint later.