Re: [PATCH V11 08/10] arm64/perf: Add struct brbe_regset helper functions

[Anshuman Khandual <anshuman.khandual@arm.com>]

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 ?

> 
> 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 ?