linux-kernel - Re: [PATCH 1/1] Perf: fix overwritten of free running PEBSv3 timestamp

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Message-ID: <20160211145702.GK6357@twins.programming.kicks-ass.net>
Date:	Thu, 11 Feb 2016 15:57:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tong Zhang <ztong@...edu>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] Perf: fix overwritten of free running PEBSv3
 timestamp

On Wed, Feb 10, 2016 at 05:38:02PM -0500, Tong Zhang wrote:
>  commit a7b58d211ba18c9175b139e18b68c86a6bcc3c3f introduced feature of
>  timestamp on free running PEBSv3, however, the timestamp is later
>  overwritten, which makes the effort in vain.
> 
> This patch fixed this problem by detecting whether timestamp is provided.
> 
> Signed-off-by: Tong Zhang <ztong@...edu>
> ---
>  include/linux/perf_event.h  |  1 +
>  kernel/events/core.c        | 45 +++++++++++++++++++++++++++++++++------------
>  kernel/events/ring_buffer.c |  4 +++-
>  3 files changed, 37 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index f9828a4..12d7b95 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -815,6 +815,7 @@ static inline void perf_sample_data_init(struct perf_sample_data *data,
>  	data->weight = 0;
>  	data->data_src.val = PERF_MEM_NA;
>  	data->txn = 0;
> +	data->time = 0;
>  }

Argh, you just touched a new cacheline and made _every_ single event
slower.