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

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Message-ID: <32cc3d3e-88df-405f-5278-7fe00d066a93@linaro.org>
Date:   Thu, 19 Oct 2017 11:15:30 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Matt Redfearn <matt.redfearn@...s.com>
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 18/10/2017 22:34, Thomas Gleixner wrote:
> 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.

Would it be worth to add some comment in include/linux/clockchips.h in
the structure definition for the different callbacks to tell which ones
are called with the irq disabled ?


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