linux-kernel - Re: [PATCH] mm: memory_hotplug: put migration failure information under DEBUG

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Message-ID: <775a56a9-b301-31bb-cd6d-8b82b1dd4d65@suse.cz>
Date:   Tue, 24 Nov 2020 14:39:42 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Charan Teja Kalla <charante@...eaurora.org>,
        Michal Hocko <mhocko@...e.com>
Cc:     akpm@...ux-foundation.org, david@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "vinmenon@...eaurora.org" <vinmenon@...eaurora.org>
Subject: Re: [PATCH] mm: memory_hotplug: put migration failure information
 under DEBUG_VM

On 11/23/20 4:10 PM, Charan Teja Kalla wrote:
> 
> Thanks Michal!
> On 11/23/2020 7:43 PM, Michal Hocko wrote:
>> On Mon 23-11-20 19:33:16, Charan Teja Reddy wrote:
>>> When the pages are failed to get isolate or migrate, the page owner
>>> information along with page info is dumped. If there are continuous
>>> failures in migration(say page is pinned) or isolation, the log buffer
>>> is simply getting flooded with the page owner information. As most of
>>> the times page info is sufficient to know the causes for failures of
>>> migration or isolation, place the page owner information under DEBUG_VM.
>> 
>> I do not see why this path is any different from others that call
>> dump_page. Page owner can add a very valuable information to debug
>> the underlying reasons for failures here. It is an opt-in debugging
>> feature which needs to be enabled explicitly. So I would argue users
>> are ready to accept a lot of data in the kernel log.
> 
> Just thinking how frequently failures can happen in those paths. In the
> memory hotplug path, we can flood the page owner logs just by making one
> page pinned. Say If it is anonymous page, the page owner information

So you say it's flooded when page_owner info is included, but not 
flooded when only the rest of __dump_page() is printed? (which is also 
not just one or two lines). That has to be very specific rate of failures.
Anyway I don't like the solution with arbitrary config option. To 
prevent flooding we generally have ratelimiting, how about that?

Also agreed with Michal that page_owner is explicitly enabled debugging 
option and if you use it in production, that's rather surprising to me, 
and possibly more rare than DEBUG_VM, which IIRC Fedora kernels use.