Re: [PATCH V11 08/10] arm64/perf: Add struct brbe_regset helper functions
From: Anshuman Khandual
Date: Sun Jun 04 2023 - 23:15:06 EST
On 6/2/23 08:10, Namhyung Kim wrote:
> On Tue, May 30, 2023 at 9:15 PM Anshuman Khandual
> <anshuman.khandual@xxxxxxx> 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@xxxxxxx>
>> Cc: Will Deacon <will@xxxxxxxxxx>
>> Cc: Mark Rutland <mark.rutland@xxxxxxx>
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Tested-by: James Clark <james.clark@xxxxxxx>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>> ---
>
> [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 ?