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From: Leo Yan <leo.yan@linaro.org>
To: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will@kernel.org>,
	John Ogness <jogness@linutronix.de>,
	"Sebastian A. Siewior" <bigeasy@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] time/sched_clock: Use raw_read_seqcount_latch()
Date: Wed, 15 Jul 2020 23:29:26 +0800
Message-ID: <20200715152926.GA16686@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <20200715115901.515956-1-a.darwish@linutronix.de>

Hi Peter, Ahmed,

On Wed, Jul 15, 2020 at 01:59:01PM +0200, Ahmed S. Darwish wrote:
> sched_clock uses seqcount_t latching to switch between two storage
> places protected by the sequence counter. This allows it to have
> interruptible, NMI-safe, seqcount_t write side critical sections.
> 
> Since 7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()"),
> raw_read_seqcount_latch() became the standardized way for seqcount_t
> latch read paths. Due to the dependent load, it also has one read
> memory barrier less than the currently used raw_read_seqcount() API.
> 
> Use raw_read_seqcount_latch() for the seqcount_t latch read path.
> 
> Link: https://lkml.kernel.org/r/20200625085745.GD117543@hirez.programming.kicks-ass.net
> Link: https://lkml.kernel.org/r/20200715092345.GA231464@debian-buster-darwi.lab.linutronix.de
> References: 1809bfa44e10 ("timers, sched/clock: Avoid deadlock during read from NMI")
> Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
> ---
>  kernel/time/sched_clock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
> index fa3f800d7d76..ea007928d681 100644
> --- a/kernel/time/sched_clock.c
> +++ b/kernel/time/sched_clock.c
> @@ -100,7 +100,7 @@ unsigned long long notrace sched_clock(void)
>  	struct clock_read_data *rd;
> 
>  	do {
> -		seq = raw_read_seqcount(&cd.seq);
> +		seq = raw_read_seqcount_latch(&cd.seq);

Understand this is doing the same thing with __ktime_get_fast_ns() and
I saw Peter acked to make change for this.

Just want to confirm, since this patch introduces conflict with the
patch set "arm64: perf: Proper cap_user_time* support" [1], I should
rebase the patch set on top of this patch, right?

Thanks,
Leo

[1] https://patchwork.kernel.org/cover/11664031/

>  		rd = cd.read_data + (seq & 1);
> 
>  		cyc = (rd->read_sched_clock() - rd->epoch_cyc) &
> --
> 2.20.1

  reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-15  2:05 [PATCH v2 0/6] arm64: perf: Proper cap_user_time* support Leo Yan
2020-07-15  2:05 ` [PATCH v2 1/6] sched_clock: Expose struct clock_read_data Leo Yan
2020-07-15  5:56   ` Ahmed S. Darwish
2020-07-15  6:54     ` Leo Yan
2020-07-15  7:21       ` Ahmed S. Darwish
2020-07-15  8:12     ` Peter Zijlstra
2020-07-15  8:14       ` peterz
2020-07-15  9:23         ` Ahmed S. Darwish
2020-07-15  9:52           ` Peter Zijlstra
2020-07-15 11:59             ` [PATCH] time/sched_clock: Use raw_read_seqcount_latch() Ahmed S. Darwish
2020-07-15 15:29               ` Leo Yan [this message]
2020-07-15 15:58                 ` Peter Zijlstra
2020-07-16  5:22                   ` Leo Yan
2020-09-10 15:08           ` [tip: locking/core] time/sched_clock: Use raw_read_seqcount_latch() during suspend tip-bot2 for Ahmed S. Darwish
2020-07-15  2:05 ` [PATCH v2 2/6] arm64: perf: Implement correct cap_user_time Leo Yan
2020-07-15  8:38   ` Peter Zijlstra
2020-07-15 15:39     ` Leo Yan
2020-07-15  2:05 ` [PATCH v2 3/6] arm64: perf: Only advertise cap_user_time for arch_timer Leo Yan
2020-07-15  2:05 ` [PATCH v2 4/6] perf: Add perf_event_mmap_page::cap_user_time_short ABI Leo Yan
2020-07-15  2:05 ` [PATCH v2 5/6] arm64: perf: Add cap_user_time_short Leo Yan
2020-07-15  2:05 ` [PATCH v2 6/6] tools headers UAPI: Update tools's copy of linux/perf_event.h Leo Yan
2020-07-15  5:17 ` [PATCH v2 0/6] arm64: perf: Proper cap_user_time* support Ahmed S. Darwish
2020-07-15  6:29   ` Leo Yan

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