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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Juergen Gross <jgross@suse.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Jork Loeser <Jork.Loeser@microsoft.com>,
	Ingo Molnar <mingo@redhat.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Thomas Gleixner <tglx@linutronix.de>,
	KY Srinivasan <kys@microsoft.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH] x86: enable RCU based table free when PARAVIRT
Date: Thu, 24 Aug 2017 10:47:41 +0200	[thread overview]
Message-ID: <871so1tkr6.fsf__1839.46389374814$1503564531$gmane$org@vitty.brq.redhat.com> (raw)
In-Reply-To: <CA+55aFy5M-0Ru-SMoyymewffawyMs8cS2TzjodyjbAtXBp24xw@mail.gmail.com> (Linus Torvalds's message of "Wed, 23 Aug 2017 16:03:53 -0700")

Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Wed, Aug 23, 2017 at 3:36 PM, Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
>>
>> Below is test cases that allocates a lot of page tables and measuare
>> fork/exit time. (I'm not entirely sure it's the best way to stress the
>> codepath.)
>
> Looks ok to me. Doing a profile (without the RCU freeing, obviously) gives me
>
>    0.77%  a.out    [kernel.vmlinux]  [k] free_pgd_range
>
>                                           ▒
>
> so it does seem to spend time in the page directory code.
>
>> Unpatched:      average 4.8322s, stddev 0.114s
>> Patched:        average 4.8362s, stddev 0.111s
>
> Ok, I vote for avoiding the complexity of two different behaviors, and
> just making the page table freeing use RCU unconditionally.

Thanks Linus & Kirill,

I actually did a microbenchmark with mmap/munmap too but wasn't able
to see any measurable performace difference.

>
> If actively trying to trigger that code doesn't show a real measurable
> difference, I don't think it matters, and the fewer different code
> paths we have, the better.

I'll send v2 enabling HAVE_RCU_TABLE_FREE on x86 unconditionally, thanks!

-- 
  Vitaly

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  reply	other threads:[~2017-08-24  8:47 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-23 13:45 [PATCH] x86: enable RCU based table free when PARAVIRT Vitaly Kuznetsov
2017-08-23 18:26 ` Linus Torvalds
2017-08-23 18:26 ` Linus Torvalds
2017-08-23 19:59   ` Kirill A. Shutemov
2017-08-23 20:27     ` Linus Torvalds
2017-08-23 20:27     ` Linus Torvalds
2017-08-23 22:36       ` Kirill A. Shutemov
2017-08-23 22:36       ` Kirill A. Shutemov
2017-08-23 23:03         ` Linus Torvalds
2017-08-23 23:03         ` Linus Torvalds
2017-08-24  8:47           ` Vitaly Kuznetsov [this message]
2017-08-24  8:47           ` Vitaly Kuznetsov
2017-08-24  8:47           ` Kirill A. Shutemov
2017-08-24  8:47           ` Kirill A. Shutemov
2017-08-23 19:59   ` Kirill A. Shutemov
2017-08-23 13:45 Vitaly Kuznetsov

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