Re: [PATCH RFC net-next] net/sched: act_police: add support for packet-per-second policing

On Wed, 27 Jan 2021 12:02:23 +0100 Simon Horman wrote:

quoted

quoted

+void psched_ppscfg_precompute(struct psched_pktrate *r,
+			      u64 pktrate64)
+{
+	memset(r, 0, sizeof(*r));
+	r->rate_pkts_ps = pktrate64;
+	r->mult = 1;
+	/* The deal here is to replace a divide by a reciprocal one
+	 * in fast path (a reciprocal divide is a multiply and a shift)
+	 *
+	 * Normal formula would be :
+	 *  time_in_ns = (NSEC_PER_SEC * pkt_num) / pktrate64
+	 *
+	 * We compute mult/shift to use instead :
+	 *  time_in_ns = (len * mult) >> shift;
+	 *
+	 * We try to get the highest possible mult value for accuracy,
+	 * but have to make sure no overflows will ever happen.
+	 */
+	if (r->rate_pkts_ps > 0) {
+		u64 factor = NSEC_PER_SEC;
+
+		for (;;) {
+			r->mult = div64_u64(factor, r->rate_pkts_ps);
+			if (r->mult & (1U << 31) || factor & (1ULL << 63))
+				break;
+			factor <<= 1;
+			r->shift++;  

Aren't there helpers somewhere for the reciprocal divide
pre-calculation?  

Now that you mention it, yes.

Looking over reciprocal_divide() I don't think it a good fit here as it
operates on 32bit values, whereas the packet rate is 64 bit.

Packet rate could be changed to a 32 bit entity if we convince ourselves we
don't want more than 2^32 - 1 packets per second (a plausible position
IMHO) - but that leads us to a secondary issue.

The code above is very similar to an existing (long existing)
byte rate variant of this helper - psched_ratecfg_precompute().
And I do think we want to:
a) Support 64-bit byte rates. Indeed such support seems to have
   been added to support 25G use-cases
b) Calculate byte and packet rates the same way

So I feel less and less that reciprocal_divide() is a good fit.
But perhaps I am mistaken.

In the meantime I will take a look to see if a helper common function can
be made to do (64 bit) reciprocal divides for the packet and byte rate
use-cases.  I.e. the common code in psched_ppscfg_precompute() and
psched_ratecfg_precompute().

No strong feelings, I'll just ask to document the reasoning in the
commit message or the comment above.