linux-kernel - Re: [RFC PATCH 00/12] x86/mm: Implement lockless pgd_alloc()/pgd

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Message-ID: <20150611141353.GA9447@gmail.com>
Date:	Thu, 11 Jun 2015 16:13:53 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [RFC PATCH 00/12] x86/mm: Implement lockless
 pgd_alloc()/pgd_free()


[ I fat-fingered the linux-mm Cc:, so every reply will bounce on that,
  sorry about that  :-/ Fixed it in this mail's Cc: list. ]

* Ingo Molnar <mingo@...nel.org> wrote:

> Waiman Long reported 'pgd_lock' contention on high CPU count systems and 
> proposed moving pgd_lock on a separate cacheline to eliminate false sharing and 
> to reduce some of the lock bouncing overhead.

So 'pgd_lock' is a global lock, used for every new task creation:

arch/x86/mm/fault.c:DEFINE_SPINLOCK(pgd_lock);

which with a sufficiently high CPU count starts to hurt.

Thanks,

	Ingo
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