linux-kernel - Re: [PATCHv2] perf x86_64: Fix rsp register for system call fast path

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Message-ID: <20121002160626.GA26598@krava.brq.redhat.com>
Date:	Tue, 2 Oct 2012 18:06:26 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCHv2] perf x86_64: Fix rsp register for system call fast path

On Tue, Oct 02, 2012 at 05:49:26PM +0200, Frederic Weisbecker wrote:
> On Tue, Oct 02, 2012 at 04:58:15PM +0200, Jiri Olsa wrote:
> > diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> > index 915b876..11d62ff 100644
> > --- a/arch/x86/kernel/cpu/perf_event.c
> > +++ b/arch/x86/kernel/cpu/perf_event.c
> > @@ -34,6 +34,7 @@
> >  #include <asm/timer.h>
> >  #include <asm/desc.h>
> >  #include <asm/ldt.h>
> > +#include <asm/syscall.h>
> >  
> >  #include "perf_event.h"
> >  
> > @@ -1699,6 +1700,52 @@ void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
> >  	userpg->time_offset = this_cpu_read(cyc2ns_offset) - now;
> >  }
> >  
> > +#ifdef CONFIG_X86_64
> > +__weak int arch_sample_regs_user(struct pt_regs *oregs, struct pt_regs *regs)
> > +{
> > +	int kernel = !user_mode(regs);
> > +
> > +	if (kernel) {
> > +		if (current->mm)
> > +			regs = task_pt_regs(current);
> > +		else
> > +			regs = NULL;
> > +	}
> 
> Shouldn't the above stay in generic code?

could be.. I guess I thought that having the regs retrieval
plus the fixup at the same place feels better/compact ;)

but could change that if needed

SNIP

> 
> That said, a race is there already: if the syscall is interrupted before
> SAVE_ARGS and co.

yep

> 
> I'm trying to scratch my head to find a solution to detect the race and
> bail out instead of recording erroneous values but I can't find one.
> 
> Anyway this is still better than what we have now.
> 
> Another solution could be to force syscall slow path and have some variable
> set there that tells us we are in a syscall and every regs have been saved.
> 
> But we probably don't want to force syscall slow path...

I was trying something like that as well, but the one I sent looks
far less hacky to me.. :)

jirka
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