linux-kernel - Re: [PATCH v4 2/2] ThunderX2: Add Cavium ThunderX2 SoC UNCORE PMU driver

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Date:   Mon, 21 May 2018 11:37:12 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Ganapatrao Kulkarni <gklkml16@...il.com>
Cc:     Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Will Deacon <Will.Deacon@....com>, jnair@...iumnetworks.com,
        Robert Richter <Robert.Richter@...ium.com>,
        Vadim.Lomovtsev@...ium.com, Jan.Glauber@...ium.com
Subject: Re: [PATCH v4 2/2] ThunderX2: Add Cavium ThunderX2 SoC UNCORE PMU
 driver

Hi Ganapat,


Sorry for the delay in replying; I was away most of last week.

On Tue, May 15, 2018 at 04:03:19PM +0530, Ganapatrao Kulkarni wrote:
> On Sat, May 5, 2018 at 12:16 AM, Ganapatrao Kulkarni <gklkml16@...il.com> wrote:
> > On Thu, Apr 26, 2018 at 4:29 PM, Mark Rutland <mark.rutland@....com> wrote:
> >> On Wed, Apr 25, 2018 at 02:30:47PM +0530, Ganapatrao Kulkarni wrote:

> >>> +static int alloc_counter(struct thunderx2_pmu_uncore_channel *pmu_uncore)
> >>> +{
> >>> +     int counter;
> >>> +
> >>> +     raw_spin_lock(&pmu_uncore->lock);
> >>> +     counter = find_first_zero_bit(pmu_uncore->counter_mask,
> >>> +                             pmu_uncore->uncore_dev->max_counters);
> >>> +     if (counter == pmu_uncore->uncore_dev->max_counters) {
> >>> +             raw_spin_unlock(&pmu_uncore->lock);
> >>> +             return -ENOSPC;
> >>> +     }
> >>> +     set_bit(counter, pmu_uncore->counter_mask);
> >>> +     raw_spin_unlock(&pmu_uncore->lock);
> >>> +     return counter;
> >>> +}
> >>> +
> >>> +static void free_counter(struct thunderx2_pmu_uncore_channel *pmu_uncore,
> >>> +                                     int counter)
> >>> +{
> >>> +     raw_spin_lock(&pmu_uncore->lock);
> >>> +     clear_bit(counter, pmu_uncore->counter_mask);
> >>> +     raw_spin_unlock(&pmu_uncore->lock);
> >>> +}
> >>
> >> I don't believe that locking is required in either of these, as the perf
> >> core serializes pmu::add() and pmu::del(), where these get called.
> 
> without this locking, i am seeing "BUG: scheduling while atomic" when
> i run perf with more events together than the maximum counters
> supported

Did you manage to get to the bottom of this?

Do you have a backtrace?

It looks like in your latest posting you reserve counters through the
userspace ABI, which doesn't seem right to me, and I'd like to
understand the problem.

Thanks,
Mark.