linux-kernel - Re: [RFC v2 29/34] mm: slub: Move flush_cpu_slab() invocations __free

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Message-ID: <ea99ecc5-365d-95d5-df88-e32a07b67b3a@suse.cz>
Date:   Thu, 10 Jun 2021 10:32:14 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [RFC v2 29/34] mm: slub: Move flush_cpu_slab() invocations
 __free_slab() invocations out of IRQ context

On 6/10/21 12:29 AM, Cyrill Gorcunov wrote:
> On Wed, Jun 09, 2021 at 01:38:58PM +0200, Vlastimil Babka wrote:
>> +static DEFINE_MUTEX(flush_lock);
>> +static DEFINE_PER_CPU(struct slub_flush_work, slub_flush);
>> +
>>  static void flush_all(struct kmem_cache *s)
>>  {
>> -	on_each_cpu_cond(has_cpu_slab, flush_cpu_slab, s, 1);
>> +	struct slub_flush_work *sfw;
>> +	unsigned int cpu;
>> +
>> +	cpus_read_lock();
>> +	mutex_lock(&flush_lock);
>> +
> 
> Hi, Vlastimil! Could you please point why do you lock cpus first and
> mutex only after? Why not mutex_lock + cpus_read_lock instead?

Good question! I must admit I didn't think about it much and just followed the
order that was in the original Sebastian's patch [1]
But there was a good reason for this order as some paths via
__kmem_cache_shutdown() and __kmem_cache_shrink() were alreadu called under
cpus_read_lock. Meanwhile mainline (me, actually) removed those, so now it
doesn't seem to be a need to keep this order anymore and we could switch it.

Thanks,
Vlastimil

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/patches/0005-mm-slub-Move-flush_cpu_slab-invocations-__free_slab-.patch?h=linux-5.12.y-rt-patches