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From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	X86 ML <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath
Date: Tue, 10 Mar 2015 14:26:06 +0100	[thread overview]
Message-ID: <20150310132606.GC26185@gmail.com> (raw)
In-Reply-To: <CALCETrWMiyJxaUKVFG-sm_HeQeq0Z7JitSEK64ZZXxgN8NSmig@mail.gmail.com>


* Andy Lutomirski <luto@amacapital.net> wrote:

> > Since this patch does add two extra MOVs,
> > I did benchmark these patches. They add exactly one cycle
> > to system call code path on my Sandy Bridge CPU.
> 
> Personally, I'm willing to pay that cycle.  It could be a bigger 
> savings on context switch, and the simplification it enables is 
> pretty good.

But, but ... context switches are a relative slow path, compared to 
system calls. And I say this with the scheduler maintainer hat on as 
well.

So this is not a good bargain IMHO, assuming it's not some _huge_ 
difference in maintainability - but having an extra percpu field
isn't really much of a problem.

I don't claim that we couldn't in some other situation decide that a 
certain type of speedup isn't worth it - but what's the big problem 
here? A bit of arithmetics shouldn't be a problem?

Thanks,

	Ingo

  reply	other threads:[~2015-03-10 13:26 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-09 18:39 [PATCH 0/4 v2] x86: entry_64.S: steps towards simpler iret frame handling Denys Vlasenko
2015-03-09 18:39 ` [PATCH 1/4] x86: save r11 into pt_regs->flags on SYSCALL64 fastpath Denys Vlasenko
2015-03-09 20:02   ` Andy Lutomirski
2015-03-16 12:04   ` [tip:x86/asm] x86/asm/entry/64: Save R11 into pt_regs-> flags " tip-bot for Denys Vlasenko
2015-03-09 18:39 ` [PATCH 3/4] x86: save user rsp in pt_regs->sp " Denys Vlasenko
2015-03-09 20:11   ` Andy Lutomirski
2015-03-09 20:32     ` Denys Vlasenko
2015-03-09 20:43       ` Andy Lutomirski
2015-03-10 12:51   ` Ingo Molnar
2015-03-10 13:10     ` Andy Lutomirski
2015-03-10 13:18     ` Denys Vlasenko
2015-03-10 13:20       ` Andy Lutomirski
2015-03-10 13:26         ` Ingo Molnar [this message]
2015-03-10 13:21       ` Ingo Molnar
2015-03-10 13:26         ` Andy Lutomirski
2015-03-10 14:00           ` Denys Vlasenko
2015-03-10 14:02             ` Andy Lutomirski
2015-03-10 14:09               ` Denys Vlasenko
2015-03-10 13:28         ` Ingo Molnar
2015-03-10 13:50         ` Denys Vlasenko
2015-03-16  9:44           ` Ingo Molnar
2015-03-16 12:05   ` [tip:x86/asm] x86/asm/entry/64: Save user RSP in pt_regs-> sp " tip-bot for Denys Vlasenko
2015-03-10  6:00 ` [PATCH 0/4 v2] x86: entry_64.S: steps towards simpler iret frame handling Ingo Molnar

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