From 4725344462362e2ce2645f354737a8ea4280fa57 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Sat, 29 Nov 2025 09:57:40 +0000 Subject: [PATCH] time/timecounter: Inline timecounter_cyc2time() New network transport protocols want NIC drivers to get hardware timestamps of all incoming packets, and possibly all outgoing packets. One example is the upcoming 'Swift congestion control' which is used by TCP transport and is the primary need for timecounter_cyc2time(). This means timecounter_cyc2time() can be called more than 100 million times per second on a busy server. Inlining timecounter_cyc2time() brings a 12% improvement on a UDP receive stress test on a 100Gbit NIC. Note that FDO, LTO, PGO are unable to magically help for this case, presumably because NIC drivers are almost exclusively shipped as modules. Add an unlikely() around the cc_cyc2ns_backwards() case, even if FDO (when used) is able to take care of this optimization. Signed-off-by: Eric Dumazet Signed-off-by: Thomas Gleixner Link: https://research.google/pubs/swift-delay-is-simple-and-effective-for-congestion-control-in-the-datacenter/ Link: https://patch.msgid.link/20251129095740.3338476-1-edumazet@google.com --- include/linux/timecounter.h | 31 +++++++++++++++++++++++++++++-- kernel/time/timecounter.c | 35 ----------------------------------- 2 files changed, 29 insertions(+), 37 deletions(-) diff --git a/include/linux/timecounter.h b/include/linux/timecounter.h index dce03a5cafb7c..7de6b350e5590 100644 --- a/include/linux/timecounter.h +++ b/include/linux/timecounter.h @@ -115,6 +115,15 @@ extern void timecounter_init(struct timecounter *tc, */ extern u64 timecounter_read(struct timecounter *tc); +/* + * This is like cyclecounter_cyc2ns(), but it is used for computing a + * time previous to the time stored in the cycle counter. + */ +static inline u64 cc_cyc2ns_backwards(const struct cyclecounter *cc, u64 cycles, u64 frac) +{ + return ((cycles * cc->mult) - frac) >> cc->shift; +} + /** * timecounter_cyc2time - convert a cycle counter to same * time base as values returned by @@ -131,7 +140,25 @@ extern u64 timecounter_read(struct timecounter *tc); * * Returns: cycle counter converted to nanoseconds since the initial time stamp */ -extern u64 timecounter_cyc2time(const struct timecounter *tc, - u64 cycle_tstamp); +static inline u64 timecounter_cyc2time(const struct timecounter *tc, u64 cycle_tstamp) +{ + const struct cyclecounter *cc = tc->cc; + u64 delta = (cycle_tstamp - tc->cycle_last) & cc->mask; + u64 nsec = tc->nsec, frac = tc->frac; + + /* + * Instead of always treating cycle_tstamp as more recent than + * tc->cycle_last, detect when it is too far in the future and + * treat it as old time stamp instead. + */ + if (unlikely(delta > cc->mask / 2)) { + delta = (tc->cycle_last - cycle_tstamp) & cc->mask; + nsec -= cc_cyc2ns_backwards(cc, delta, frac); + } else { + nsec += cyclecounter_cyc2ns(cc, delta, tc->mask, &frac); + } + + return nsec; +} #endif diff --git a/kernel/time/timecounter.c b/kernel/time/timecounter.c index 3d2a354cfe1c1..2e64dbb6302d7 100644 --- a/kernel/time/timecounter.c +++ b/kernel/time/timecounter.c @@ -62,38 +62,3 @@ u64 timecounter_read(struct timecounter *tc) } EXPORT_SYMBOL_GPL(timecounter_read); -/* - * This is like cyclecounter_cyc2ns(), but it is used for computing a - * time previous to the time stored in the cycle counter. - */ -static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc, - u64 cycles, u64 mask, u64 frac) -{ - u64 ns = (u64) cycles; - - ns = ((ns * cc->mult) - frac) >> cc->shift; - - return ns; -} - -u64 timecounter_cyc2time(const struct timecounter *tc, - u64 cycle_tstamp) -{ - u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask; - u64 nsec = tc->nsec, frac = tc->frac; - - /* - * Instead of always treating cycle_tstamp as more recent - * than tc->cycle_last, detect when it is too far in the - * future and treat it as old time stamp instead. - */ - if (delta > tc->cc->mask / 2) { - delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask; - nsec -= cc_cyc2ns_backwards(tc->cc, delta, tc->mask, frac); - } else { - nsec += cyclecounter_cyc2ns(tc->cc, delta, tc->mask, &frac); - } - - return nsec; -} -EXPORT_SYMBOL_GPL(timecounter_cyc2time); -- 2.47.3