linux-kernel - Re: [PATCH 1/3] clocksource/mips-gic-timer: Fix rcu

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Date:   Thu, 19 Oct 2017 11:09:02 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Matt Redfearn <matt.redfearn@...s.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-mips@...ux-mips.org,
        Matt Redfearn <matt.redfearn@...tec.com>,
        "# v3 . 19 +" <stable@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] clocksource/mips-gic-timer: Fix rcu_sched timeouts
 from multithreading

On 11/10/2017 16:01, Matt Redfearn wrote:
> When the MIPS GIC clockevent code was written, it appears to have
> inherited the 0x300 cycle min delta from the MIPS CPU timer driver. This
> is suboptimal for two reasons.
> 
> Firstly, the CPU timer counts once every other cycle (i.e. half the
> clock rate). The GIC counts once per clock. Assuming that the GIC and
> CPU share the same clock this means the GIC is counting twice as fast,
> and so the min delta should be (at least) doubled. Fix this by doubling
> the min delta to 0x600.
> 
> Secondly, the fixed min delta ignores the fact that with MIPS
> multithreading active, execution resource within a core is shared
> between the hardware threads within that core. An inconvenienly timed
> switch of executing thread within gic_next_event, between the read and
> write of updated count, can result in the CPU writing an event in the
> past, and subsequently not receiving a tick interrupt until the counter
> wraps. This stalls the CPU from the RCU scheduler. Other CPUs detect
> this and print rcu_sched timeout messages in  the kernel log. It can
> lead to other issues as well if the CPU is holding locks or other
> resources at the point at which it stalls. Fix this by scaling the min
> delta for the timer based on the number of threads in the core
> (smp_num_siblings). This accounts for the greater average runtime of
> CPUs within a multithreading core.
> 
> Signed-off-by: Matt Redfearn <matt.redfearn@...tec.com>
> Fixes: b695d8e6ad6f ("clocksource: mips-gic: Use clockevents_config_and_register")
> Cc: <stable@...r.kernel.org> # v3.19 +
> 
> Signed-off-by: Matt Redfearn <matt.redfearn@...s.com>
> ---

Matt,

I'm dropping your series.

The first patch fails to compile because of the smp_num_siblings
variable undefined when compile testing on another arch (probably a
header is missing).

The issue the patch address could be fixed in the time framework as
stated by Thomas.

As spotted by Thomas also, the local_irq_save() is not needed in the
set_next_event() function, so the second patch is pointless and a patch
removing those local_irq_save()/restore() would make more sense.

The third patch does not longer apply after removing the two above.

 -- Daniel

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