without runtime.Gosched

null:00   ; oid=0..16995  nread=68269354  t=536.560158ms (31.569µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=532.416867ms (31.326µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=536.958977ms (31.593µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=534.170594ms (31.429µs / object)  x=zsha1.go

with runtime.Gosched

null:00   ; oid=0..16995  nread=68269354  t=594.966346ms (35.006µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=597.510359ms (35.155µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=598.251026ms (35.199µs / object)  x=zsha1.go
null:00   ; oid=0..16995  nread=68269354  t=596.02138ms (35.068µs / object)  x=zsha1.go

----

-> trace shows runtime.Gosched indeed switches to woken up G without
second syscall in serveRecv but serveRecv migrates to different M (so
different CPU)