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From: Davidlohr Bueso <dave@stgolabs.net>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Michal Hocko <mhocko@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu rwsem
Date: Tue, 4 Jul 2017 08:42:08 -0700	[thread overview]
Message-ID: <20170704154208.GC11168@linux-80c1.suse> (raw)
In-Reply-To: <alpine.DEB.2.20.1707041732030.9000@nanos>

On Tue, 04 Jul 2017, Thomas Gleixner wrote:

>On Tue, 4 Jul 2017, Davidlohr Bueso wrote:
>> On Tue, 04 Jul 2017, Davidlohr Bueso wrote:
>>
>> > As a side effect you end up optimizing get/put_online_mems() at the cost
>> > of more overhead for the actual hotplug operation, which is rare and of less
>> > performance importance.
>>
>> So nm this, the reader side actually gets _more_ expensive with pcpu-rwsems
>> due to at least two full barriers for each get/put operation.
>
>Compared to a mutex_lock/unlock() pair on a global mutex ....

Ah, right, I was thrown off the:

    if (mem_hotplug.active_writer == current)
       return;

checks, which is only true within hotplug_begin/end. So normally we'd take
the lock, which was my first impression. Sorry for the noise.

Thanks,
Davidlohr

WARNING: multiple messages have this Message-ID (diff)
From: Davidlohr Bueso <dave@stgolabs.net>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Michal Hocko <mhocko@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu rwsem
Date: Tue, 4 Jul 2017 08:42:08 -0700	[thread overview]
Message-ID: <20170704154208.GC11168@linux-80c1.suse> (raw)
In-Reply-To: <alpine.DEB.2.20.1707041732030.9000@nanos>

On Tue, 04 Jul 2017, Thomas Gleixner wrote:

>On Tue, 4 Jul 2017, Davidlohr Bueso wrote:
>> On Tue, 04 Jul 2017, Davidlohr Bueso wrote:
>>
>> > As a side effect you end up optimizing get/put_online_mems() at the cost
>> > of more overhead for the actual hotplug operation, which is rare and of less
>> > performance importance.
>>
>> So nm this, the reader side actually gets _more_ expensive with pcpu-rwsems
>> due to at least two full barriers for each get/put operation.
>
>Compared to a mutex_lock/unlock() pair on a global mutex ....

Ah, right, I was thrown off the:

    if (mem_hotplug.active_writer == current)
       return;

checks, which is only true within hotplug_begin/end. So normally we'd take
the lock, which was my first impression. Sorry for the noise.

Thanks,
Davidlohr

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  reply	other threads:[~2017-07-04 15:42 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-07-04  9:32 [patch V2 0/2] mm/memory_hotplug: Cure potential deadlocks vs. cpu hotplug lock Thomas Gleixner
2017-07-04  9:32 ` Thomas Gleixner
2017-07-04  9:32 ` [patch V2 1/2] mm: swap: Provide lru_add_drain_all_cpuslocked() Thomas Gleixner
2017-07-04  9:32   ` Thomas Gleixner
2017-07-04 10:58   ` Michal Hocko
2017-07-04 10:58     ` Michal Hocko
2017-07-04 12:48     ` Thomas Gleixner
2017-07-04 12:48       ` Thomas Gleixner
2017-07-04 12:52       ` Michal Hocko
2017-07-04 12:52         ` Michal Hocko
2017-07-04 12:07   ` Vlastimil Babka
2017-07-04 12:07     ` Vlastimil Babka
2017-07-04 12:35     ` Thomas Gleixner
2017-07-04 12:35       ` Thomas Gleixner
2017-07-04  9:32 ` [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu rwsem Thomas Gleixner
2017-07-04  9:32   ` Thomas Gleixner
2017-07-04 10:59   ` Michal Hocko
2017-07-04 10:59     ` Michal Hocko
2017-07-04 12:10   ` Vlastimil Babka
2017-07-04 12:10     ` Vlastimil Babka
2017-07-04 12:49     ` Thomas Gleixner
2017-07-04 12:49       ` Thomas Gleixner
2017-07-04 15:01   ` Davidlohr Bueso
2017-07-04 15:01     ` Davidlohr Bueso
2017-07-04 15:22     ` Davidlohr Bueso
2017-07-04 15:22       ` Davidlohr Bueso
2017-07-04 15:32       ` Thomas Gleixner
2017-07-04 15:32         ` Thomas Gleixner
2017-07-04 15:42         ` Davidlohr Bueso [this message]
2017-07-04 15:42           ` Davidlohr Bueso
2017-07-05 21:53 ` [patch V2 0/2] mm/memory_hotplug: Cure potential deadlocks vs. cpu hotplug lock Andrew Morton
2017-07-05 21:53   ` Andrew Morton
2017-07-06  6:34   ` Thomas Gleixner
2017-07-06  6:34     ` Thomas Gleixner

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