linux-kernel - Re: [PATCH 2/2 --resend] perf: update userspace page info for software event

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Date:	Wed, 11 Feb 2015 12:15:33 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Shaohua Li <shli@...com>
Cc:	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 2/2 --resend] perf: update userspace page info for
 software event

On Thu, Feb 05, 2015 at 03:55:32PM -0800, Shaohua Li wrote:
> For hardware event, the userspace page of the event gets updated in
> context switch, so if we read time in the page, we get updated info. For
> software event, this is missed currently. This patch makes the behavior
> consistency.
> 
> With this patch, we can implement clock_gettime(THREAD_CPUTIME) with
> PERF_COUNT_SW_DUMMY in userspace as suggested by Andy and Peter. Code
> likes this:
> 
>   if (pc->cap_user_time) {
> 	do {
> 		seq = pc->lock;
> 		barrier();
> 
> 		running = pc->time_running;
> 		cyc = rdtsc();
> 		time_mult = pc->time_mult;
> 		time_shift = pc->time_shift;
> 		time_offset = pc->time_offset;
> 
> 		barrier();
> 	} while (pc->lock != seq);
> 
> 	quot = (cyc >> time_shift);
> 	rem = cyc & ((1 << time_shift) - 1);
> 	delta = time_offset + quot * time_mult +
> 		((rem * time_mult) >> time_shift);

You could maybe use:

static inline u64 mul_u64_u32_shr(u64 a, u32 mul, unsigned int shift)
{
        return (u64)(((unsigned __int128)a * mul) >> shift);
}

And save yourself a mult instruction if you have suitable (64bit)
hardware and a recent GCC.

> 	running += delta;
> 	return running;
>   }
> 

Thanks for poking me. Applied.
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