linux-kernel - Re: [PATCH 2/3] perf/arm-cmn: Rework DTC counters (again)

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Message-ID: <8e179525-bba-c577-85cf-4aa0a7af436@os.amperecomputing.com>
Date:   Fri, 20 Oct 2023 15:50:30 -0700 (PDT)
From:   Ilkka Koskinen <ilkka@...amperecomputing.com>
To:     Robin Murphy <robin.murphy@....com>
cc:     will@...nel.org, mark.rutland@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        jeremy.linton@....com, ilkka@...amperecomputing.com,
        renyu.zj@...ux.alibaba.com
Subject: Re: [PATCH 2/3] perf/arm-cmn: Rework DTC counters (again)


Hi Robin,

I have one comment, otherwise the patch looks good to me.

On Fri, 20 Oct 2023, Robin Murphy wrote:
> The bitmap-based scheme for tracking DTC counter usage turns out to be a
> complete dead-end for its imagined purpose, since by the time we have to
> keep track of a per-DTC counter index anyway, we already have enough
> information to make the bitmap itself redundant. Revert the remains of
> it back to almost the original scheme, but now expanded to track per-DTC
> indices, in preparation for making use of them in anger.
>
> Note that since cycle count events always use a dedicated counter on a
> single DTC, we reuse the field to encode their DTC index directly.
>
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
> drivers/perf/arm-cmn.c | 126 +++++++++++++++++++++--------------------
> 1 file changed, 64 insertions(+), 62 deletions(-)
>
> diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> index f1ac8d0cdb3b..675f1638013e 100644
> --- a/drivers/perf/arm-cmn.c
> +++ b/drivers/perf/arm-cmn.c
> @@ -281,16 +281,13 @@ struct arm_cmn_node {
> 	u16 id, logid;
> 	enum cmn_node_type type;
>

....

> @@ -589,8 +586,7 @@ static void arm_cmn_debugfs_init(struct arm_cmn *cmn, int id) {}
> struct arm_cmn_hw_event {
> 	struct arm_cmn_node *dn;
> 	u64 dtm_idx[4];
> -	unsigned int dtc_idx;
> -	u8 dtcs_used;
> +	s8 dtc_idx[CMN_MAX_DTCS];
> 	u8 num_dns;
> 	u8 dtm_offset;
> 	bool wide_sel;
> @@ -600,6 +596,10 @@ struct arm_cmn_hw_event {
> #define for_each_hw_dn(hw, dn, i) \
> 	for (i = 0, dn = hw->dn; i < hw->num_dns; i++, dn++)
>
> +/* @i is the DTC number, @idx is the counter index on that DTC */
> +#define for_each_hw_dtc_idx(hw, i, idx) \
> +	for (int i = 0, idx; i < CMN_MAX_DTCS; i++) if ((idx = hw->dtc_idx[i]) >= 0)

Isn't that "idx" unnecessary in the initialization?

Cheers, Ilkka