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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Vlastimil Babka <vbabka@suse.cz>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@redhat.com>,
        akpm@linux-foundation.org, frederic@kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, cl@linux.com,
        juri.lelli@redhat.com, mingo@redhat.com, mtosatti@redhat.com,
        nilal@redhat.com, mgorman@suse.de, ppandit@redhat.com,
        williams@redhat.com, bigeasy@linutronix.de,
        anna-maria@linutronix.de, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH 0/6] mm: Remote LRU per-cpu pagevec cache/per-cpu page
 list drain support
In-Reply-To: <efb4a5e1-d64a-38de-97d8-350c3d1b0962@suse.cz>
References: <20210921161323.607817-1-nsaenzju@redhat.com>
 <f608b4bf-aa36-b0c4-e748-4f39010f3d06@suse.cz>
 <20210922112817.GO4323@worktop.programming.kicks-ass.net>
 <87v92sgt3n.ffs@tglx> <efb4a5e1-d64a-38de-97d8-350c3d1b0962@suse.cz>
Date:   Thu, 23 Sep 2021 12:36:37 +0200
Message-ID: <878rznh93e.ffs@tglx>
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Vlastimil,

On Thu, Sep 23 2021 at 09:12, Vlastimil Babka wrote:
> On 9/23/21 00:09, Thomas Gleixner wrote:
>>   local_lock() 		-> preempt_disable()
>>   local_lock_irq()	-> local_irq_disable()
>>   ...
>
> Yes, to be clean, this would have to be a new primitive, not just an abused
> local lock. It would just look similar to the RT version (a percpu array of
> spinlocks), so for this patchset it would allow not to have two such locks
> side be side (local + spin) while only one is being used. For maximum
> flexibility the initialization would take a CONFIG_ (or something
> compile-time bool) that when false would make the !RT version an empty
> struct and "locking" would rely on preempt/irq disable (just as with !RT
> local_lock). If compile-time true it would take a static key to decide on
> boot whether the !RT version only does the preepmt/irq disable or actually
> takes the lock.
>
> But as you say below, it's too much complexity for questionable benefit.
>
> But maybe this can all be avoided anyway, as I recalled what we do for
> vmstat already (IIUC). See quiet_vmstat() - when cpu enters the nohz mode,
> it flushes per-cpu vmstat diffs and then there's no reason to further
> disturb the cpu to do that while it's on NOHZ mode. We could do the same for
> lru pagevecs and pcplists?

I'm not sure about this. I like the idea of being able to offload things
to housekeeping CPUs not only in the full isolation case.

A good example is RCU which allows to offload all RCU processing to some
other CPU(s), which is useful even w/o full isolation.

The synchronous quiescing on entering NOHZ full mode is a cute
workaround but for one it makes entering NOHZ full more expensive and it
does not necessarily provide good isolation guarantees under all
circumstances, while a full remote processing definitely does.

I think it's at least worthwhile to investigate.

Thanks,

        tglx