Date:   Tue, 22 Feb 2022 13:27:22 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     K Prateek Nayak <kprateek.nayak@....com>
Cc:     peterz@...radead.org, aubrey.li@...ux.intel.com, efault@....de,
        gautham.shenoy@....com, linux-kernel@...r.kernel.org,
        mingo@...nel.org, song.bao.hua@...ilicon.com,
        srikar@...ux.vnet.ibm.com, valentin.schneider@....com,
        vincent.guittot@...aro.org
Subject: Re: [PATCH v5] sched/fair: Consider cpu affinity when allowing NUMA
 imbalance in find_idlest_group

On Tue, Feb 22, 2022 at 03:51:33PM +0530, K Prateek Nayak wrote:
> In the case of systems containing multiple LLCs per socket, like
> AMD Zen systems, users want to spread bandwidth hungry applications
> across multiple LLCs. Stream is one such representative workload where
> the best performance is obtained by limiting one stream thread per LLC.
> To ensure this, users are known to pin the tasks to a specify a subset
> of the CPUs consisting of one CPU per LLC while running such bandwidth
> hungry tasks.
> 
> Suppose we kickstart a multi-threaded task like stream with 8 threads
> using taskset or numactl to run on a subset of CPUs on a 2 socket Zen3
> server where each socket contains 128 CPUs
> (0-63,128-191 in one socket, 64-127,192-255 in another socket)
> 
> Eg: numactl -C 0,16,32,48,64,80,96,112 ./stream8
> 
> Here each CPU in the list is from a different LLC and 4 of those LLCs
> are on one socket, while the other 4 are on another socket.
> 
> Ideally we would prefer that each stream thread runs on a different
> CPU from the allowed list of CPUs. However, the current heuristics in
> find_idlest_group() do not allow this during the initial placement.
> 
> Suppose the first socket (0-63,128-191) is our local group from which
> we are kickstarting the stream tasks. The first four stream threads
> will be placed in this socket. When it comes to placing the 5th
> thread, all the allowed CPUs are from the local group (0,16,32,48)
> would have been taken.
> 
> However, the current scheduler code simply checks if the number of
> tasks in the local group is fewer than the allowed numa-imbalance
> threshold. This threshold was previously 25% of the NUMA domain span
> (in this case threshold = 32) but after the v6 of Mel's patchset
> "Adjust NUMA imbalance for multiple LLCs", got merged in sched-tip,
> Commit: e496132ebedd ("sched/fair: Adjust the allowed NUMA imbalance
> when SD_NUMA spans multiple LLCs") it is now equal to number of LLCs
> in the NUMA domain, for processors with multiple LLCs.
> (in this case threshold = 8).
> 
> For this example, the number of tasks will always be within threshold
> and thus all the 8 stream threads will be woken up on the first socket
> thereby resulting in sub-optimal performance.
> 
> The following sched_wakeup_new tracepoint output shows the initial
> placement of tasks in the current tip/sched/core on the Zen3 machine:
> 
> stream-5045    [032] d..2.   167.914699: sched_wakeup_new: comm=stream pid=5047 prio=120 target_cpu=048
> stream-5045    [032] d..2.   167.914746: sched_wakeup_new: comm=stream pid=5048 prio=120 target_cpu=000
> stream-5045    [032] d..2.   167.914846: sched_wakeup_new: comm=stream pid=5049 prio=120 target_cpu=016
> stream-5045    [032] d..2.   167.914891: sched_wakeup_new: comm=stream pid=5050 prio=120 target_cpu=032
> stream-5045    [032] d..2.   167.914928: sched_wakeup_new: comm=stream pid=5051 prio=120 target_cpu=032
> stream-5045    [032] d..2.   167.914976: sched_wakeup_new: comm=stream pid=5052 prio=120 target_cpu=032
> stream-5045    [032] d..2.   167.915011: sched_wakeup_new: comm=stream pid=5053 prio=120 target_cpu=032
> 
> Once the first four threads are distributed among the allowed CPUs of
> socket one, the rest of the treads start piling on these same CPUs
> when clearly there are CPUs on the second socket that can be used.
> 
> Following the initial pile up on a small number of CPUs, though the
> load-balancer eventually kicks in, it takes a while to get to {4}{4}
> and even {4}{4} isn't stable as we observe a bunch of ping ponging
> between {4}{4} to {5}{3} and back before a stable state is reached
> much later (1 Stream thread per allowed CPU) and no more migration is
> required.
> 
> We can detect this piling and avoid it by checking if the number of
> allowed CPUs in the local group are fewer than the number of tasks
> running in the local group and use this information to spread the
> 5th task out into the next socket (after all, the goal in this
> slowpath is to find the idlest group and the idlest CPU during the
> initial placement!).
> 
> The following sched_wakeup_new tracepoint output shows the initial
> placement of tasks after adding this fix on the Zen3 machine:
> 
> stream-4733    [032] d..2.   116.017980: sched_wakeup_new: comm=stream pid=4735 prio=120 target_cpu=048
> stream-4733    [032] d..2.   116.018032: sched_wakeup_new: comm=stream pid=4736 prio=120 target_cpu=000
> stream-4733    [032] d..2.   116.018127: sched_wakeup_new: comm=stream pid=4737 prio=120 target_cpu=064
> stream-4733    [032] d..2.   116.018185: sched_wakeup_new: comm=stream pid=4738 prio=120 target_cpu=112
> stream-4733    [032] d..2.   116.018235: sched_wakeup_new: comm=stream pid=4739 prio=120 target_cpu=096
> stream-4733    [032] d..2.   116.018289: sched_wakeup_new: comm=stream pid=4740 prio=120 target_cpu=016
> stream-4733    [032] d..2.   116.018334: sched_wakeup_new: comm=stream pid=4741 prio=120 target_cpu=080
> 
> We see that threads are using all of the allowed CPUs and there is
> no pileup.
> 
> No output is generated for tracepoint sched_migrate_task with this
> patch due to a perfect initial placement which removes the need
> for balancing later on - both across NUMA boundaries and within
> NUMA boundaries for stream.
> 
> Following are the results from running 8 Stream threads with and
> without pinning on a dual socket Zen3 Machine (2 x 64C/128T):
> 
> Pinning is done using: numactl -C 0,16,32,48,64,80,96,112 ./stream8
> 
> 	           5.17.0-rc1               5.17.0-rc1                5.17.0-rc1
>                tip sched/core           tip sched/core            tip sched/core
>                  (no pinning)                 +pinning              + this-patch
> 								       + pinning
> 
>  Copy:    97699.28 (0.00 pct)     95933.60  (-1.80 pct)    156578.91 (60.26 pct)
> Scale:   107754.15 (0.00 pct)     91869.88 (-14.74 pct)    149783.25 (39.00 pct)
>   Add:   126383.29 (0.00 pct)    105730.86 (-16.34 pct)    186493.09 (47.56 pct)
> Triad:   124896.78 (0.00 pct)    106394.38 (-14.81 pct)    184733.48 (47.90 pct)
> 
> Pinning currently hurts the performance compared to unbound case on
> tip/sched/core. With the addition of this patch, we are able to
> outperform tip/sched/core by a good margin with pinning.
> 
> Following are the results from running 16 Stream threads with and
> without pinning on a dual socket Skylake Machine (2 x 24C/48T):
> 
> Pinning is done using: numactl -C 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 ./stream16
> 
> 	           5.17.0-rc1               5.17.0-rc1                5.17.0-rc1
>                tip sched/core           tip sched/core            tip sched/core
>                  (no pinning)                 +pinning              + this-patch
> 								       + pinning
> 
>  Copy:   126620.67 (0.00 pct)     141062.10 (11.40 pct)    147615.44 (16.58 pct)
> Scale:   91313.51 (0.00 pct)      112879.61 (23.61 pct)    122591.28 (34.25 pct)
>   Add:   102035.43 (0.00 pct)     125889.98 (23.37 pct)    138179.01 (35.42 pct)
> Triad:   102281.91 (0.00 pct)     123743.48 (20.98 pct)    138940.41 (35.84 pct)
> 
> In case of Skylake machine, with single LLC per socket, we see good
> improvement brought about by pinning which is further benefited by
> this patch.
> 
> Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>

Only minor nit would that the cpumask can be declared within the if
block to limit scope but that is just being picky so

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

Thanks!

-- 
Mel Gorman
SUSE Labs

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