linux-ext4 - Re: Kernel Benchmarking

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Date:   Tue, 15 Sep 2020 10:28:50 -0400
From:   "Chris Mason" <clm@...com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Dave Chinner <david@...morbit.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Michael Larabel <Michael@...haellarabel.com>,
        Ted Ts'o <tytso@...gle.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: Kernel Benchmarking

On 13 Sep 2020, at 23:31, Matthew Wilcox wrote:

> On Mon, Sep 14, 2020 at 09:45:03AM +1000, Dave Chinner wrote:
>> I have my doubts that complex page cache manipulation operations
>> like ->migrate_page that rely exclusively on page and internal mm
>> serialisation are really safe against ->fallocate based invalidation
>> races.  I think they probably also need to be wrapped in the
>> MMAPLOCK, but I don't understand all the locking and constraints
>> that ->migrate_page has and there's been no evidence yet that it's a
>> problem so I've kinda left that alone. I suspect that "no evidence"
>> thing comes from "filesystem people are largely unable to induce
>> page migrations in regression testing" so it has pretty much zero
>> test coverage....
>
> Maybe we can get someone who knows the page migration code to give
> us a hack to induce pretty much constant migration?

While debugging migrate page problems, I usually run dbench and

while(true) ; do echo 1 > /proc/sys/vm/compact_memory ; done

I’ll do this with a mixture of memory pressure or drop_caches or a 
memory hog depending on what I hope to trigger.

Because of hugepage allocations, we tend to bash on migration/compaction 
fairly hard in the fleet.  We do fallocate in some of these workloads as 
well, but I’m sure it doesn’t count as complete coverage for the 
races Dave is worried about.

-chris