linux-kernel - Re: [PATCH v4 08/11] drivers: perf: hisi: use poll method to avoid L3C counter overflow

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Date:   Fri, 24 Feb 2017 08:41:10 +0530
From:   Anurup M <anurupvasu@...il.com>
To:     Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        anurup.m@...wei.com, zhangshaokun@...ilicon.com,
        tanxiaojun@...wei.com, xuwei5@...ilicon.com,
        sanil.kumar@...ilicon.com, john.garry@...wei.com,
        gabriele.paoloni@...wei.com, shiju.jose@...wei.com,
        huangdaode@...ilicon.com, linuxarm@...wei.com,
        dikshit.n@...wei.com, shyju.pv@...wei.com
Subject: Re: [PATCH v4 08/11] drivers: perf: hisi: use poll method to avoid
 L3C counter overflow



On Tuesday 21 February 2017 05:39 PM, Will Deacon wrote:
> On Mon, Feb 20, 2017 at 11:09:43AM +0000, Mark Rutland wrote:
>> On Sun, Feb 19, 2017 at 01:51:03PM -0500, Anurup M wrote:
>>> The L3 cache PMU use N-N SPI interrupt which has no support
>>> in kernel mainline.
>> Could you elaborate on what you mean by this?
>>
>> I don't understand what is meant here. How exactly are the interrupts
>> wired up in HW, and what exactly is not supported by Linux?
>>
>>> So use hrtimer to poll and update event
>>> counter to avoid overflow condition for L3 cache PMU.
>>> A interval of 10 seconds is used for the hrtimer.
>>> The time interval can be configured in the sysfs.
>> I'm not too keen on giving userspace the ability to control this, since
>> it gives an awful lot of rope for userspace to tie around itself.
> Agreed. I'd also go a step further and say that for PMUs with either
> terminally broken interrupts (like this one) or just missing interrupts
> (like the CPU PMU on raspberry pi iirc), then the perf core should take
> care of an hrtimer in an attempt to generate samples often enough. We
> already have PERF_PMU_CAP_NO_INTERRUPT, but it currently just disables
> sampling events.
>
> The fiddly part is knowing how to program the timer, and I think you'd
> need the PMU driver to provide an upper-bound on events per nanosecond.
> I'm pretty sure that would be highly unreliable (especially for shared
> resources such as the L3), at which point, is it worth the hassle?

Agreed, it is difficult for user to arrive at a interval for the shared 
resource
like L3 cache. So I shall remove this facility exposed to user.

Shall use a realistic and safer upper bound as hrtimer interval for the 
uncore
units which do not support IRQ.

Thanks,
Anuurp

> Will