runtime: execute memory barrier conditionally when changing netpoll timers

We only need the memory barrier in poll_runtime_pollSetDeadline only
when one of the timers has fired, which is not the expected case.
Memory barrier can be somewhat expensive on some archs,
so execute it only if one of the timers has in fact fired.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.0µs ± 0%  17.1µs ± 0%  +0.35%  (p=0.032 n=5+5)
SetReadDeadline-6      232ns ± 0%   230ns ± 0%  -1.03%  (p=0.000 n=4+5)

Update #25729

Change-Id: Ifce6f505b9e7ba3717bad8f454077a2e94ea6e75
Reviewed-on: https://go-review.googlesource.com/c/146343
Reviewed-by: Ian Lance Taylor <iant@golang.org>

diff --git a/src/runtime/netpoll.go b/src/runtime/netpoll.go
index 34e7c902ebca1ff13f4e91ff959a9ee41be18160..f914844cdf5084d777ffe13ce58d9db7928eedf8 100644 (file)
--- a/src/runtime/netpoll.go
+++ b/src/runtime/netpoll.go
@@ -254,12 +254,14 @@ func poll_runtime_pollSetDeadline(pd *pollDesc, d int64, mode int) {
        }
        // If we set the new deadline in the past, unblock currently pending IO if any.
        var rg, wg *g
-       atomicstorep(unsafe.Pointer(&wg), nil) // full memory barrier between stores to rd/wd and load of rg/wg in netpollunblock
-       if pd.rd < 0 {
-               rg = netpollunblock(pd, 'r', false)
-       }
-       if pd.wd < 0 {
-               wg = netpollunblock(pd, 'w', false)
+       if pd.rd < 0 || pd.wd < 0 {
+               atomicstorep(unsafe.Pointer(&wg), nil) // full memory barrier between stores to rd/wd and load of rg/wg in netpollunblock
+               if pd.rd < 0 {
+                       rg = netpollunblock(pd, 'r', false)
+               }
+               if pd.wd < 0 {
+                       wg = netpollunblock(pd, 'w', false)
+               }
        }
        unlock(&pd.lock)
        if rg != nil {