linux-kernel - Re: [PATCH] x86/mm: Tracking linear mapping split events since boot

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Message-ID: <YA9tk7Tnp2AEEcCU@slm.duckdns.org>
Date:   Mon, 25 Jan 2021 20:17:07 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Saravanan D <saravanand@...look.com>, x86@...nel.org,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] x86/mm: Tracking linear mapping split events since boot

Hello,

On Mon, Jan 25, 2021 at 05:04:00PM -0800, Dave Hansen wrote:
> Which part?  Large production environments don't trust data from
> debugfs?  Or don't trust it if it might have been reset?

When the last reset was. Not saying it's impossible or anything but in
general it's a lot better to have the counters to be monotonically
increasing with time/event stamped markers than the counters themselves
getting reset or modified in other ways because the ownership of a specific
counter might not be obvious to everyone and accidents and mistakes happen.

Note that the "time/event stamped markers" above don't need to and shouldn't
be in the kernel. It can be managed by whoever that wants to monitor a given
time period and there can be any number of them.

> You could stick the "reset" switch in debugfs, and dump something out in
> dmesg like we do for /proc/sys/vm/drop_caches so it's not a surprise
> that it happened.

Processing dmesgs can work too but isn't particularly reliable or scalable.

> BTW, counts of *events* don't really belong in meminfo.  These really do
> belong in /proc/vmstat if anything.

Oh yeah, I don't have a strong opinion on where the counters should go.

Thanks.

-- 
tejun