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

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Date:   Wed, 18 Oct 2017 22:34:50 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Matt Redfearn <matt.redfearn@...s.com>
cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        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 Wed, 11 Oct 2017, 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.

I don't understand why this is not catched by the check at the end of the
next_event() function:

        res = ((int)(gic_read_count() - cnt) >= 0) ? -ETIME : 0;

Btw, the local_irq_save() in this function is pointless as this function is
always called with interrupts disabled from the core code.

Thanks,

	tglx