Re: [PATCH V11 08/10] arm64/perf: Add struct brbe_regset helper functions
From: Namhyung Kim
Date: Mon Jun 05 2023 - 19:49:32 EST
On Sun, Jun 4, 2023 at 8:15 PM Anshuman Khandual
<anshuman.khandual@xxxxxxx> wrote:
>
>
>
> 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 ?
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