Re: [PATCH V12 08/10] arm64/perf: Add struct brbe_regset helper functions

[Mark Rutland <mark.rutland@arm.com>]

Hi Anshuman,

Thanks, this is looking much better; I just a have a couple of minor comments.

With those fixed up:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

On Thu, Jun 15, 2023 at 07:02:37PM +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@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>
> ---
>  drivers/perf/arm_brbe.c | 127 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 127 insertions(+)
> 
> diff --git a/drivers/perf/arm_brbe.c b/drivers/perf/arm_brbe.c
> index 4729cb49282b..f6693699fade 100644
> --- a/drivers/perf/arm_brbe.c
> +++ b/drivers/perf/arm_brbe.c
> @@ -44,6 +44,133 @@ static void select_brbe_bank(int bank)
>  	isb();
>  }
>  
> +static bool __read_brbe_regset(struct brbe_regset *entry, int idx)
> +{
> +	entry->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.
> +	 */

This comment refers to the process of handling multiple entries, though it's
only handling one entry, and I don't think we need to mention saving cycles here.

Could we please delete this comment entirely? The comment above
capture_brbe_regset() already explains that we read until the first invalid
entry.

> +	if (brbe_invalid(entry->brbinf))
> +		return false;
> +
> +	entry->brbsrc = get_brbsrc_reg(idx);
> +	entry->brbtgt = get_brbtgt_reg(idx);
> +	return true;
> +}
> +
> +/*
> + * This scans over BRBE register banks and captures individual branch records
> + * [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.
> + */

Could we simplify this to:

/*
 * Read all BRBE entries in HW until the first invalid entry.
 *
 * The caller must ensure that the BRBE is not concurrently modifying these
 * entries.
 */

> +static int capture_brbe_regset(int nr_hw_entries, struct brbe_regset *buf)
> +{
> +	int idx = 0;
> +
> +	select_brbe_bank(BRBE_BANK_IDX_0);
> +	while (idx < nr_hw_entries && idx < BRBE_BANK0_IDX_MAX) {
> +		if (!__read_brbe_regset(&buf[idx], idx))
> +			return idx;
> +		idx++;
> +	}
> +
> +	select_brbe_bank(BRBE_BANK_IDX_1);
> +	while (idx < nr_hw_entries && idx < BRBE_BANK1_IDX_MAX) {
> +		if (!__read_brbe_regset(&buf[idx], idx))
> +			return idx;
> +		idx++;
> +	}
> +	return idx;
> +}
> +
> +/*
> + * 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 records. Unless the live buffer is detected 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_move = min(nr_stored, nr_max - nr_live);
> +
> +	/* 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);
> +}
> +
>  /*
>   * Generic perf branch filters supported on BRBE
>   *
> -- 
> 2.25.1
> 

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