Date: Fri, 7 Aug 2015 16:30:12 +0200
From: Michal Hocko <mhocko@kernel.org>
To: PINTU KUMAR <pintu.k@samsung.com>
Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, minchan@kernel.org, dave@stgolabs.net,
        koct9i@gmail.com, mgorman@suse.de, vbabka@suse.cz, js1304@gmail.com,
        hannes@cmpxchg.org, alexander.h.duyck@redhat.com,
        sasha.levin@oracle.com, cl@linux.com, fengguang.wu@intel.com,
        cpgs@samsung.com, pintu_agarwal@yahoo.com, pintu.k@outlook.com,
        vishnu.ps@samsung.com, rohit.kr@samsung.com
Subject: Re: [PATCH 1/1] mm: vmstat: introducing vm counter for slowpath
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On Fri 07-08-15 18:16:47, PINTU KUMAR wrote:
[...]
> > On Fri 07-08-15 12:38:54, Pintu Kumar wrote:
> > > This patch add new counter slowpath_entered in /proc/vmstat to track
> > > how many times the system entered into slowpath after first allocation
> > > attempt is failed.
> > 
> > This is too lowlevel to be exported in the regular user visible interface IMO.
> > 
> I think its ok because I think this interface is for lowlevel debugging itself.

Yes but this might change in future implementations where the counter
might be misleading or even lacking any meaning. This is a user visible
interface which has to be maintained practically for ever. We have made
those mistakes in the past...

[...]
> This information is good for kernel developers.

Then make it a trace point and you can dump even more information. E.g.
timestamps, gfp_mask, order...

[...]

> Regarding trace points, I am not sure if we can attach counter to it.

You do not need to have a counter. You just watch for the tracepoint
while debugging your particular problem.

> Also trace may have more over-head 

Tracepoints should be close to 0 overhead when disabled and certainly
not a performance killer during the debugging session.

> and requires additional configs to be enabled to debug.

This is to be expected for the debugging sessions. And I am pretty
sure that the static event tracepoints do not require anything really
excessive.

> Mostly these configs will not be enabled by default (at least in embedded, low
> memory device).

Are you sure? I thought that CONFIG_TRACING should be sufficient for
EVENT_TRACING but I am not familiar with this too deeply...
-- 
Michal Hocko
SUSE Labs
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