Re: [PATCH 1/1] mm: vmstat: introducing vm counter for slowpath

[Michal Hocko <mhocko@kernel.org>]

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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