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Date:   Wed, 14 Jun 2023 11:59:28 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Anshuman Khandual <anshuman.khandual@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        will@kernel.org, catalin.marinas@arm.com,
        Mark Brown <broonie@kernel.org>,
        James Clark <james.clark@arm.com>,
        Rob Herring <robh@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Suzuki Poulose <suzuki.poulose@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        linux-perf-users@vger.kernel.org
Subject: Re: [PATCH V11 08/10] arm64/perf: Add struct brbe_regset helper
 functions
Message-ID: <ZImdkHxE6kX+dT5e@FVFF77S0Q05N>
References: <20230531040428.501523-1-anshuman.khandual@arm.com>
 <20230531040428.501523-9-anshuman.khandual@arm.com>
 <ZIikv0bW1xGeLf6B@FVFF77S0Q05N.cambridge.arm.com>
 <2e91f218-e740-5b7d-fa8d-8fc43a6502a2@arm.com>
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On Wed, Jun 14, 2023 at 10:44:38AM +0530, Anshuman Khandual wrote:
> On 6/13/23 22:47, Mark Rutland wrote:
> >> +/*
> >> + * 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.

Yes, my bad.

> >> +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 ?

Yup, my bad again. That should be min().

> > 	/* 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.

Great!

Thanks,
Mark.