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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Mon, 5 Jun 2023 16:49:14 -0700
Message-ID: <CAM9d7cgzEJgNWvXn32cj82GaAvn3CPhE1PyLr+FtFfrauj0X6g@mail.gmail.com>
Subject: Re: [PATCH V11 08/10] arm64/perf: Add struct brbe_regset helper functions
To:     Anshuman Khandual <anshuman.khandual@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        will@kernel.org, catalin.marinas@arm.com, mark.rutland@arm.com,
        Mark Brown <broonie@kernel.org>,
        James Clark <james.clark@arm.com>,
        Rob Herring <robh@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Suzuki Poulose <suzuki.poulose@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        linux-perf-users@vger.kernel.org
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On Sun, Jun 4, 2023 at 8:15 PM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
>
>
>
> On 6/2/23 08:10, Namhyung Kim wrote:
> > On Tue, May 30, 2023 at 9:15 PM Anshuman Khandual
> > <anshuman.khandual@arm.com> wrote:
> >>
> >> The primary abstraction level for fetching branch records from BRBE HW has
> >> been changed as 'struct brbe_regset', which contains storage for all three
> >> BRBE registers i.e BRBSRC, BRBTGT, BRBINF. Whether branch record processing
> >> happens in the task sched out path, or in the PMU IRQ handling path, these
> >> registers need to be extracted from the HW. Afterwards both live and stored
> >> sets need to be stitched together to create final branch records set. This
> >> adds required helper functions for such operations.
> >>
> >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >> Cc: Will Deacon <will@kernel.org>
> >> Cc: Mark Rutland <mark.rutland@arm.com>
> >> Cc: linux-arm-kernel@lists.infradead.org
> >> Cc: linux-kernel@vger.kernel.org
> >> Tested-by: James Clark <james.clark@arm.com>
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> >> ---
> >
> > [SNIP]
> >> +
> >> +static inline void copy_brbe_regset(struct brbe_regset *src, int src_idx,
> >> +                                   struct brbe_regset *dst, int dst_idx)
> >> +{
> >> +       dst[dst_idx].brbinf = src[src_idx].brbinf;
> >> +       dst[dst_idx].brbsrc = src[src_idx].brbsrc;
> >> +       dst[dst_idx].brbtgt = src[src_idx].brbtgt;
> >> +}
> >> +
> >> +/*
> >> + * This function concatenates branch records from stored and live buffer
> >> + * up to maximum nr_max records and the stored buffer holds the resultant
> >> + * buffer. The concatenated buffer contains all the branch records from
> >> + * the live buffer but might contain some from stored buffer considering
> >> + * the maximum combined length does not exceed 'nr_max'.
> >> + *
> >> + *     Stored records  Live records
> >> + *     ------------------------------------------------^
> >> + *     |       S0      |       L0      |       Newest  |
> >> + *     ---------------------------------               |
> >> + *     |       S1      |       L1      |               |
> >> + *     ---------------------------------               |
> >> + *     |       S2      |       L2      |               |
> >> + *     ---------------------------------               |
> >> + *     |       S3      |       L3      |               |
> >> + *     ---------------------------------               |
> >> + *     |       S4      |       L4      |               nr_max
> >> + *     ---------------------------------               |
> >> + *     |               |       L5      |               |
> >> + *     ---------------------------------               |
> >> + *     |               |       L6      |               |
> >> + *     ---------------------------------               |
> >> + *     |               |       L7      |               |
> >> + *     ---------------------------------               |
> >> + *     |               |               |               |
> >> + *     ---------------------------------               |
> >> + *     |               |               |       Oldest  |
> >> + *     ------------------------------------------------V
> >> + *
> >> + *
> >> + * S0 is the newest in the stored records, where as L7 is the oldest in
> >> + * the live reocords. Unless the live buffer is detetcted as being full
> >> + * thus potentially dropping off some older records, L7 and S0 records
> >> + * are contiguous in time for a user task context. The stitched buffer
> >> + * here represents maximum possible branch records, contiguous in time.
> >> + *
> >> + *     Stored records  Live records
> >> + *     ------------------------------------------------^
> >> + *     |       L0      |       L0      |       Newest  |
> >> + *     ---------------------------------               |
> >> + *     |       L0      |       L1      |               |
> >> + *     ---------------------------------               |
> >> + *     |       L2      |       L2      |               |
> >> + *     ---------------------------------               |
> >> + *     |       L3      |       L3      |               |
> >> + *     ---------------------------------               |
> >> + *     |       L4      |       L4      |             nr_max
> >> + *     ---------------------------------               |
> >> + *     |       L5      |       L5      |               |
> >> + *     ---------------------------------               |
> >> + *     |       L6      |       L6      |               |
> >> + *     ---------------------------------               |
> >> + *     |       L7      |       L7      |               |
> >> + *     ---------------------------------               |
> >> + *     |       S0      |               |               |
> >> + *     ---------------------------------               |
> >> + *     |       S1      |               |    Oldest     |
> >> + *     ------------------------------------------------V
> >> + *     |       S2      | <----|
> >> + *     -----------------      |
> >> + *     |       S3      | <----| Dropped off after nr_max
> >> + *     -----------------      |
> >> + *     |       S4      | <----|
> >> + *     -----------------
> >> + */
> >> +static int stitch_stored_live_entries(struct brbe_regset *stored,
> >> +                                     struct brbe_regset *live,
> >> +                                     int nr_stored, int nr_live,
> >> +                                     int nr_max)
> >> +{
> >> +       int nr_total, nr_excess, nr_last, i;
> >> +
> >> +       nr_total = nr_stored + nr_live;
> >> +       nr_excess = nr_total - nr_max;
> >> +
> >> +       /* Stored branch records in stitched buffer */
> >> +       if (nr_live == nr_max)
> >> +               nr_stored = 0;
> >> +       else if (nr_excess > 0)
> >> +               nr_stored -= nr_excess;
> >> +
> >> +       /* Stitched buffer branch records length */
> >> +       if (nr_total > nr_max)
> >> +               nr_last = nr_max;
> >> +       else
> >> +               nr_last = nr_total;
> >> +
> >> +       /* Move stored branch records */
> >> +       for (i = 0; i < nr_stored; i++)
> >> +               copy_brbe_regset(stored, i, stored, nr_last - nr_stored - 1 + i);
> >
> > I'm afraid it can overwrite some entries if nr_live is small
> > and nr_stored is big.  Why not use memmove()?
>
> nr_stored is first adjusted with nr_excess if both live and stored entries combined
> exceed the maximum branch records in the HW. I am wondering how it can override ?

Say nr_stored = 40 and nr_live = 20, wouldn't it copy stored[0] to stored[20]?
Then stored[20:39] will be lost.  Also I'm not sure "-1" is correct.

>
> >
> > Also I think it'd be simpler if you copy store to live.
> > It'll save copying live in the IRQ but it will copy the
> > whole content to store again for the sched switch.
>
> But how that is better than the current scheme ?

I guess normally the live buffer is full, then it can skip
the copy and use the buffer directly for IRQ, right?

Thanks,
Namhyung