linux-kernel - Re: [PATCH 1/2] Revert "mm: don't reclaim inodes with many attached pages"

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Date:   Thu, 31 Jan 2019 15:48:11 +0000
From:   Chris Mason <clm@...com>
To:     Dave Chinner <david@...morbit.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        Roman Gushchin <guro@...com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "vdavydov.dev@...il.com" <vdavydov.dev@...il.com>
Subject: Re: [PATCH 1/2] Revert "mm: don't reclaim inodes with many attached
 pages"

On 30 Jan 2019, at 20:34, Dave Chinner wrote:

> On Wed, Jan 30, 2019 at 12:21:07PM +0000, Chris Mason wrote:
>>
>>
>> On 29 Jan 2019, at 23:17, Dave Chinner wrote:
>>
>>> From: Dave Chinner <dchinner@...hat.com>
>>>
>>> This reverts commit a76cf1a474d7dbcd9336b5f5afb0162baa142cf0.
>>>
>>> This change causes serious changes to page cache and inode cache
>>> behaviour and balance, resulting in major performance regressions
>>> when combining worklaods such as large file copies and kernel
>>> compiles.
>>>
>>> https://bugzilla.kernel.org/show_bug.cgi?id=202441
>>
>> I'm a little confused by the latest comment in the bz:
>>
>> https://bugzilla.kernel.org/show_bug.cgi?id=202441#c24
>
> Which says the first patch that changed the shrinker behaviour is
> the underlying cause of the regression.
>
>> Are these reverts sufficient?
>
> I think so.

Based on the latest comment:

"If I had been less strict in my testing I probably would have 
discovered that the problem was present earlier than 4.19.3. Mr Gushins 
commit made it more visible.
I'm going back to work after two days off, so I might not be able to 
respond inside your working hours, but I'll keep checking in on this as 
I get a chance."

I don't think the reverts are sufficient.

>
>> Roman beat me to suggesting Rik's followup.  We hit a different 
>> problem
>> in prod with small slabs, and have a lot of instrumentation on Rik's
>> code helping.
>
> I think that's just another nasty, expedient hack that doesn't solve
> the underlying problem. Solving the underlying problem does not
> require changing core reclaim algorithms and upsetting a page
> reclaim/shrinker balance that has been stable and worked well for
> just about everyone for years.
>

Things are definitely breaking down in non-specialized workloads, and 
have been for a long time.

-chris