Re: [PATCH 1/3] perf/arm-cmn: Decouple wp_config registers from filter group number

[Ilkka Koskinen]

Hi Robin,

Thanks for the review.

On Mon, 29 Jan 2024, Robin Murphy wrote:
> Hi Ilkka,
>
> On 2024-01-26 10:12 pm, Ilkka Koskinen wrote:
> > Previously, wp_config0/2 registers were used for primary match group and
> > wp_config1/3 registers for secondary match group. In order to support
> > tertiary match group, this patch decouples the registers and the groups.
>
> Happy to see you having a stab at this, however I fear I you're in for a fair 
> dose of "if it were this simple I might have already done it" :)

Uh, for a little while I thought it seemed too easy but decided to 
continue nevertheless

> > Allocation is changed to dynamic but it's still per mesh instance rather
> > than per node.
> >
> > Signed-off-by: Ilkka Koskinen <ilkka@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/perf/arm-cmn.c | 52 ++++++++++++++++++++++++++++++++++--------
> >   1 file changed, 43 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> > index c584165b13ba..93eb47ea7e25 100644
> > --- a/drivers/perf/arm-cmn.c
> > +++ b/drivers/perf/arm-cmn.c
> > @@ -591,6 +591,7 @@ struct arm_cmn_hw_event {
> >   	u8 dtm_offset;
> >   	bool wide_sel;
> >   	enum cmn_filter_select filter_sel;
> > +	int wp_idx;
> >   };
> >     #define for_each_hw_dn(hw, dn, i) \
> > @@ -1337,7 +1338,35 @@ static const struct attribute_group 
> > *arm_cmn_attr_groups[] = {
> >     static int arm_cmn_wp_idx(struct perf_event *event)
> >   {
> > -	return CMN_EVENT_EVENTID(event) + CMN_EVENT_WP_GRP(event);
> > +	struct arm_cmn_hw_event *hw = to_cmn_hw(event);
> > +
> > +	return hw->wp_idx;
>
> Sorry, this breaks group validation.

Clearly, watchpoint group validation was missing from my tests :(

> > +}
> > +
> > +static int arm_cmn_wp_idx_unused(struct perf_event *event, struct 
> > arm_cmn_dtm *dtm,
> > +	struct arm_cmn_dtc *dtc)
> > +{
> > +	struct arm_cmn_hw_event *hw = to_cmn_hw(event);
> > +	int idx, tmp, direction = CMN_EVENT_EVENTID(event);
> > +
> > +	/*
> > +	 * Examine wp 0 & 1 for the up direction,
> > +	 * examine wp 2 & 3 for the down direction
> > +	 */
> > +	for (idx = direction; idx < direction + 2; idx++)
> > +		if (dtm->wp_event[idx] < 0)
> > +			break;
> > +
> > +	if (idx == direction + 2)
> > +		return -ENOSPC;
> > +
> > +	tmp = dtm->wp_event[idx ^ 1];
> > +	if (tmp >= 0 && CMN_EVENT_WP_COMBINE(event) !=
> > +	    CMN_EVENT_WP_COMBINE(dtc->counters[tmp]))
> > +		return -ENOSPC;
> > +
> > +	hw->wp_idx = idx;
>
> I don't really get this logic either - we can allocate a potentially 
> different index for every DTM, but only store the most recent one?
>
> > +	return hw->wp_idx;
> >   }
> >     static u32 arm_cmn_wp_config(struct perf_event *event)
> > @@ -1785,6 +1814,8 @@ static void arm_cmn_event_clear(struct arm_cmn *cmn, 
> > struct perf_event *event,
> >     	for_each_hw_dtc_idx(hw, j, idx)
> >   		cmn->dtc[j].counters[idx] = NULL;
> > +
> > +	hw->wp_idx = -1;
> >   }
> >     static int arm_cmn_event_add(struct perf_event *event, int flags)
> > @@ -1794,6 +1825,7 @@ static int arm_cmn_event_add(struct perf_event 
> > *event, int flags)
> >   	struct arm_cmn_node *dn;
> >   	enum cmn_node_type type = CMN_EVENT_TYPE(event);
> >   	unsigned int input_sel, i = 0;
> > +	int wp_idx;
> >     	if (type == CMN_TYPE_DTC) {
> >   		while (cmn->dtc[i].cycles)
> > @@ -1822,6 +1854,7 @@ static int arm_cmn_event_add(struct perf_event 
> > *event, int flags)
> >   	}
> >     	/* ...then the local counters to feed them */
> > +	wp_idx = -1;
>
> Oh, I guess this trying to avoid some of that issue, but I still don't think 
> it works - say we add an event targeted to XP B, which sees WP0 is free on 
> DTM B so allocates index 0; then we add another event aggregating across XPs 
> A and B, which sees WP0 is free on DTM A, allocates index 0, then goes on to 
> stomp WP0 on DTM B as well - oops.
>
> I don't think it's going to be feasible to do this without tracking the full 
> allocation state with a wp_idx bitmap in the hw_event - at least it only 
> needs to be half the size of dtm_idx, so I think there's still room.

Yeah, it was supposed to be simple and stupid version until I'd have time 
to make the allocation per node. But the more I think about this, the more
I start to believe that the bitmap solution would be the better option
right away.

I'll take a look at better solution although it might take a while as I'll 
probably get other more urgent tasks soon. If you find yourself with too 
much free time, feel free to take this task ;)

Cheers, Ilkka

> Thanks,
> Robin.
>
> >   	for_each_hw_dn(hw, dn, i) {
> >   		struct arm_cmn_dtm *dtm = &cmn->dtms[dn->dtm] + 
> > hw->dtm_offset;
> >   		unsigned int dtm_idx, shift, d = max_t(int, dn->dtc, 0);
> > @@ -1835,16 +1868,17 @@ static int arm_cmn_event_add(struct perf_event 
> > *event, int flags)
> >   		if (type == CMN_TYPE_XP) {
> >   			input_sel = CMN__PMEVCNT0_INPUT_SEL_XP + dtm_idx;
> >   		} else if (type == CMN_TYPE_WP) {
> > -			int tmp, wp_idx = arm_cmn_wp_idx(event);
> >   			u32 cfg = arm_cmn_wp_config(event);
> >   -			if (dtm->wp_event[wp_idx] >= 0)
> > -				goto free_dtms;
> > -
> > -			tmp = dtm->wp_event[wp_idx ^ 1];
> > -			if (tmp >= 0 && CMN_EVENT_WP_COMBINE(event) !=
> > - 
> > CMN_EVENT_WP_COMBINE(cmn->dtc[d].counters[tmp]))
> > -				goto free_dtms;
> > +			/*
> > +			 * wp_config register index is currently allocated 
> > per
> > +			 * mesh instance rather than per node.
> > +			 */
> > +			if (wp_idx < 0) {
> > +				wp_idx = arm_cmn_wp_idx_unused(event, dtm, 
> > &cmn->dtc[d]);
> > +				if (wp_idx < 0)
> > +					goto free_dtms;
> > +			}
> >     			input_sel = CMN__PMEVCNT0_INPUT_SEL_WP + wp_idx;
> >   			dtm->wp_event[wp_idx] = hw->dtc_idx[d];