Re: [PATCH 2/3] sched/core: Introduce SM_IDLE and an idle re-entry fast-path in __schedule()

[Chen Yu <yu.c.chen@intel.com>]

On 2024-07-10 at 09:02:09 +0000, K Prateek Nayak wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> Since commit b2a02fc43a1f ("smp: Optimize
> send_call_function_single_ipi()") an idle CPU in TIF_POLLING_NRFLAG mode
> can be pulled out of idle by setting TIF_NEED_RESCHED flag to service an
> IPI without actually sending an interrupt. Even in cases where the IPI
> handler does not queue a task on the idle CPU, do_idle() will call
> __schedule() since need_resched() returns true in these cases.
> 
> Introduce and use SM_IDLE to identify call to __schedule() from
> schedule_idle() and shorten the idle re-entry time by skipping
> pick_next_task() when nr_running is 0 and the previous task is the idle
> task.
> 
> With the SM_IDLE fast-path, the time taken to complete a fixed set of
> IPIs using ipistorm improves significantly. Following are the numbers
> from a dual socket 3rd Generation EPYC system (2 x 64C/128T) (boost on,
> C2 disabled) running ipistorm between CPU8 and CPU16:
> 
> cmdline: insmod ipistorm.ko numipi=100000 single=1 offset=8 cpulist=8 wait=1
> 
>    ==================================================================
>    Test          : ipistorm (modified)
>    Units         : Normalized runtime
>    Interpretation: Lower is better
>    Statistic     : AMean
>    ==================================================================
>    kernel:				time [pct imp]
>    tip:sched/core			1.00 [baseline]
>    tip:sched/core + SM_IDLE		0.25 [75.11%]
> 
> [ kprateek: Commit log and testing ]
> 
> Link: https://lore.kernel.org/lkml/20240615012814.GP8774@noisy.programming.kicks-ass.net/
> Not-yet-signed-off-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
>

Only with current patch applied on top of sched/core commit c793a62823d1,
a significant throughput/run-to-run variance improvement is observed
on an Intel 240 CPUs/ 2 Nodes server. C-states >= C1E are disabled,
CPU frequency governor is set to performance and turbo-boost disabled.

Without the patch(lower the better):

158490995
113086433
737869191
302454894
731262790
677283357
729767478
830949261
399824606
743681976

(Amean): 542467098
(Std):   257011706


With the patch(lower the better):
128060992
115646768
132734621
150330954
113143538
169875051
145010400
151589193
162165800
159963320

(Amean): 142852063
(Std):    18646313

I've launched full tests for schbench/hackbench/netperf/tbench
to see if there is any difference.

thanks,
Chenyu