> -----Original Message----- > From: Li, Aubrey [mailto:[email protected]] > Sent: Monday, March 22, 2021 5:37 PM > To: Song Bao Hua (Barry Song) <[email protected]>; > [email protected]; [email protected]; [email protected]; > [email protected]; [email protected]; [email protected]; > [email protected]; [email protected] > Cc: [email protected]; [email protected]; > [email protected]; xuwei (O) <[email protected]>; Zengtao (B) > <[email protected]>; [email protected]; yangyicong > <[email protected]>; Liguozhu (Kenneth) <[email protected]>; > [email protected] > Subject: [Linuxarm] Re: [PATCH] sched/fair: remove redundant test_idle_cores > for non-smt > > Hi Barry, > > On 2021/3/21 6:14, Barry Song wrote: > > update_idle_core() is only done for the case of sched_smt_present. > > but test_idle_cores() is done for all machines even those without > > smt. > > The patch looks good to me. > May I know for what case we need to keep CONFIG_SCHED_SMT for non-smt > machines?
Hi Aubrey, I think the defconfig of arm64 has always enabled CONFIG_SCHED_SMT: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/configs/defconfig it is probably true for x86 as well. I don't think Linux distribution will build a separate kernel for machines without smt. so basically the kernel depends on runtime topology parse to figure out if smt is present rather than depending on a rebuild. > > Thanks, > -Aubrey > > > > this could contribute to up 8%+ hackbench performance loss on a > > machine like kunpeng 920 which has no smt. this patch removes the > > redundant test_idle_cores() for non-smt machines. > > > > we run the below hackbench with different -g parameter from 2 to > > 14, for each different g, we run the command 10 times and get the > > average time: > > $ numactl -N 0 hackbench -p -T -l 20000 -g $1 > > > > hackbench will report the time which is needed to complete a certain > > number of messages transmissions between a certain number of tasks, > > for example: > > $ numactl -N 0 hackbench -p -T -l 20000 -g 10 > > Running in threaded mode with 10 groups using 40 file descriptors each > > (== 400 tasks) > > Each sender will pass 20000 messages of 100 bytes > > > > The below is the result of hackbench w/ and w/o this patch: > > g= 2 4 6 8 10 12 14 > > w/o: 1.8151 3.8499 5.5142 7.2491 9.0340 10.7345 12.0929 > > w/ : 1.8428 3.7436 5.4501 6.9522 8.2882 9.9535 11.3367 > > +4.1% +8.3% +7.3% +6.3% > > > > Signed-off-by: Barry Song <[email protected]> > > --- > > kernel/sched/fair.c | 8 +++++--- > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 2e2ab1e..de42a32 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6038,9 +6038,11 @@ static inline bool test_idle_cores(int cpu, bool def) > > { > > struct sched_domain_shared *sds; > > > > - sds = rcu_dereference(per_cpu(sd_llc_shared, cpu)); > > - if (sds) > > - return READ_ONCE(sds->has_idle_cores); > > + if (static_branch_likely(&sched_smt_present)) { > > + sds = rcu_dereference(per_cpu(sd_llc_shared, cpu)); > > + if (sds) > > + return READ_ONCE(sds->has_idle_cores); > > + } > > > > return def; > > } Thanks Barry
