[PATCH V11 08/10] arm64/perf: Add struct brbe_regset helper functions
Anshuman Khandual
anshuman.khandual at arm.com
Tue Jun 13 22:14:38 PDT 2023
On 6/13/23 22:47, Mark Rutland wrote:
> On Wed, May 31, 2023 at 09:34:26AM +0530, Anshuman Khandual 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 at arm.com>
>> Cc: Will Deacon <will at kernel.org>
>> Cc: Mark Rutland <mark.rutland at arm.com>
>> Cc: linux-arm-kernel at lists.infradead.org
>> Cc: linux-kernel at vger.kernel.org
>> Tested-by: James Clark <james.clark at arm.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
>> ---
>> drivers/perf/arm_brbe.c | 163 ++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 163 insertions(+)
>>
>> diff --git a/drivers/perf/arm_brbe.c b/drivers/perf/arm_brbe.c
>> index 484842d8cf3e..759db681d673 100644
>> --- a/drivers/perf/arm_brbe.c
>> +++ b/drivers/perf/arm_brbe.c
>> @@ -44,6 +44,169 @@ static void select_brbe_bank(int bank)
>> isb();
>> }
>>
>> +/*
>> + * This scans over BRBE register banks and captures individual branch reocrds
>> + * [BRBSRC, BRBTGT, BRBINF] into a pre-allocated 'struct brbe_regset' buffer,
>> + * until an invalid one gets encountered. The caller for this function needs
>> + * to ensure BRBE is an appropriate state before the records can be captured.
>> + */
>> +static int capture_brbe_regset(struct brbe_hw_attr *brbe_attr, struct brbe_regset *buf)
>> +{
>> + int loop1_idx1, loop1_idx2, loop2_idx1, loop2_idx2;
>> + int idx, count;
>> +
>> + loop1_idx1 = BRBE_BANK0_IDX_MIN;
>> + if (brbe_attr->brbe_nr <= BRBE_BANK_MAX_ENTRIES) {
>> + loop1_idx2 = brbe_attr->brbe_nr - 1;
>> + loop2_idx1 = BRBE_BANK1_IDX_MIN;
>> + loop2_idx2 = BRBE_BANK0_IDX_MAX;
>> + } else {
>> + loop1_idx2 = BRBE_BANK0_IDX_MAX;
>> + loop2_idx1 = BRBE_BANK1_IDX_MIN;
>> + loop2_idx2 = brbe_attr->brbe_nr - 1;
>> + }
>> +
>> + select_brbe_bank(BRBE_BANK_IDX_0);
>> + for (idx = 0, count = loop1_idx1; count <= loop1_idx2; idx++, count++) {
>> + buf[idx].brbinf = get_brbinf_reg(idx);
>> + /*
>> + * There are no valid entries anymore on the buffer.
>> + * Abort the branch record processing to save some
>> + * cycles and also reduce the capture/process load
>> + * for the user space as well.
>> + */
>> + if (brbe_invalid(buf[idx].brbinf))
>> + return idx;
>> +
>> + buf[idx].brbsrc = get_brbsrc_reg(idx);
>> + buf[idx].brbtgt = get_brbtgt_reg(idx);
>> + }
>> +
>> + select_brbe_bank(BRBE_BANK_IDX_1);
>> + for (count = loop2_idx1; count <= loop2_idx2; idx++, count++) {
>> + buf[idx].brbinf = get_brbinf_reg(idx);
>> + /*
>> + * There are no valid entries anymore on the buffer.
>> + * Abort the branch record processing to save some
>> + * cycles and also reduce the capture/process load
>> + * for the user space as well.
>> + */
>> + if (brbe_invalid(buf[idx].brbinf))
>> + return idx;
>> +
>> + buf[idx].brbsrc = get_brbsrc_reg(idx);
>> + buf[idx].brbtgt = get_brbtgt_reg(idx);
>> + }
>> + return idx;
>> +}
>
> As with __armv8pmu_branch_read(), the loop conditions are a bit hard to follow,
> and I believe that can be rewritten along the lines of the suggestion there.
I have changed both the places (in separate patches) with suggested loop structure.
>
> Looking at this, we now have a couple of places that will try to read the
> registers for an individual record, so it probably makes sense to facotr that
> into a helper, e.g.
There are indeed two places inside capture_brbe_regset() - one for each bank.
>
> | static bool __read_brbe_regset(struct brbe_regset *entry, int idx)
> | {
> | u64 brbinf = get_brbinf_reg(idx);
> |
> | if (brbe_invalid(brbinf))
> | return false;
> |
> | entry->brbinf = brbinf;
> | entry->brbsrc = get_brbsrc_reg(idx);
> | entry->brbtgt = get_brbtgt_reg(idx);
> |
> | return true;
> | }
>
> ... which can be used here, e.g.
>
> | /*
> | * Capture all records before the first invalid record, and return the number
> | * of records captured.
> | */
> | static int capture_brbe_regset(struct brbe_hw_attr *brbe_attr, struct brbe_regset *buf)
> | {
> |
> | int nr_entries = brbe_attr->brbe_nr;
> | int idx = 0;
> |
> | select_brbe_bank(BRBE_BANK_IDX_0);
> | while (idx < nr_entries && IDX < BRBE_BANK0_IDX_MAX) {
> | if (__read_brbe_regset(&buf[idx], idx))
It should test !_read_brbe_regset(&buf[idx], idx)) instead as the error
case returns false.
> | return idx;
> | }
> |
> | select_brbe_bank(BRBE_BANK_IDX_1);
> | while (idx < nr_entries && IDX < BRBE_BANK1_IDX_MAX) {
> | if (__read_brbe_regset(&buf[idx], idx))
> | return idx;
Ditto.
> | }
> |
> | return idx;
> | }
Will factor out a new helper __read_brbe_regset() from capture_brbe_regset().
>
> ... and could be used to implement capture_branch_entry() in the patch before
> this.
>
>> +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;
>> +}
>
> C can do struct assignment, so this is the same as:
>
> | static inline void copy_brbe_regset(struct brbe_regset *src, int src_idx,
> | struct brbe_regset *dst, int dst_idx)
> | {
> | dst[dst_idx] = src[src_idx];
> | }
Agreed.
>
> ... and given that, it would be simpler and clearer to have that directly in
> the caller, so I don't think we need this helper function.
Agreed, will drop copy_brbe_regset().
>
>> +/*
>> + * 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
Fixed these typos ^^^ ^^^
>> + * 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);
>> +
>> + /* Copy live branch records */
>> + for (i = 0; i < nr_live; i++)
>> + copy_brbe_regset(live, i, stored, i);
>> +
>> + return nr_last;
>> +}
>
> I think this can be written more simply as something like:
>
> 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_move = max(nr_stored, nr_max - nr_live);
Should this compare be min() instead ? As all nr_live entries need to
be moved starting store[0], there will be (nr_max - nr_live) entries
left for initial stored entries movement, irrespective of how many of
stored entries are actually present. Hence (nr_max - nr_live) acts as
a cap on nr_stored value for this initial movement. But if nr_stored
is smaller than nr_max - nr_live, it gets picked up.
>
> /* Move the tail of the buffer to make room for the new entries */
> memmove(&stored[nr_live], &stored[0], nr_move * sizeof(*stored));
>
> /* Copy the new entries into the head of the buffer */
> memcpy(stored[0], &live[0], nr_live * sizeof(*stored));
>
> /* Return the number of entries in the stitched buffer */
> return min(nr_live + nr_stored, nr_max);
> }
Otherwise this makes sense and simpler, will rework.
>
> ... or if we could save this oldest-first, we could make it a circular buffer
> and avoid moving older entries.
Storing the youngest entries first is aligned with how perf branch
stack sampling stores the entries in struct perf_sample_data which
gets copied 'as is' from cpuc->branches->branch_stack. Hence, just
keeping all these buffer in the same age order (youngest first in
index 0) really makes sense. Although the above rework seems fine.
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