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Date: Thu, 11 Jun 2015 16:13:53 +0200
From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC PATCH 00/12] x86/mm: Implement lockless
 pgd_alloc()/pgd_free()
Message-ID: <20150611141353.GA9447@gmail.com>
References: <1434031637-9091-1-git-send-email-mingo@kernel.org>
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Cc: Andy Lutomirski <luto@amacapital.net>, Andrew Morton <akpm@linux-foundation.org>, Denys Vlasenko <dvlasenk@redhat.com>, Brian Gerst <brgerst@gmail.com>, Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>, Linus Torvalds <torvalds@linux-foundation.org>, Oleg Nesterov <oleg@redhat.com>, Thomas Gleixner <tglx@linutronix.de>, Waiman Long <Waiman.Long@hp.com>


[ 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@kernel.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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