Re: [Patch v3 1/3] lib: Restrict cpumask_local_spread to houskeeping CPUs

[Nitesh Narayan Lal <nitesh@redhat.com>]

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On 6/24/20 8:13 AM, Frederic Weisbecker wrote:
> On Tue, Jun 23, 2020 at 03:23:29PM -0400, Nitesh Narayan Lal wrote:
>> From: Alex Belits <abelits@marvell.com>
>>
>> The current implementation of cpumask_local_spread() does not respect the
>> isolated CPUs, i.e., even if a CPU has been isolated for Real-Time task,
>> it will return it to the caller for pinning of its IRQ threads. Having
>> these unwanted IRQ threads on an isolated CPU adds up to a latency
>> overhead.
>>
>> Restrict the CPUs that are returned for spreading IRQs only to the
>> available housekeeping CPUs.
>>
>> Signed-off-by: Alex Belits <abelits@marvell.com>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
>> ---
>>  lib/cpumask.c | 16 +++++++++++-----
>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/lib/cpumask.c b/lib/cpumask.c
>> index fb22fb266f93..d73104995981 100644
>> --- a/lib/cpumask.c
>> +++ b/lib/cpumask.c
>> @@ -6,6 +6,7 @@
>>  #include <linux/export.h>
>>  #include <linux/memblock.h>
>>  #include <linux/numa.h>
>> +#include <linux/sched/isolation.h>
>>  
>>  /**
>>   * cpumask_next - get the next cpu in a cpumask
>> @@ -205,22 +206,27 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask)
>>   */
>>  unsigned int cpumask_local_spread(unsigned int i, int node)
>>  {
>> -	int cpu;
>> +	int cpu, hk_flags;
>> +	const struct cpumask *mask;
>>  
>> +	hk_flags = HK_FLAG_DOMAIN | HK_FLAG_WQ;
> This should be HK_FLAG_MANAGED_IRQ instead of HK_FLAG_WQ since this
> function seem to be used mostly to select CPUs to affine managed IRQs.

IIRC then there are drivers such as ixgbe that use cpumask_local_spread while
affining NORMAL IRQs as well.
But I can recheck that.

> In the end the cpumask you pass to IRQ core will be filtered throughout
> HK_FLAG_MANAGED_IRQ anyway so better select an appropriate one in the
> first place to avoid an empty cpumask intersection.
>
> Now even if cpumask_local_spread() is currently mostly used to select
> managed irq targets, the name and role of the function don't refer to that.
> Probably cpumask_local_spread() should take HK_ flag in parameter so that
> it can correctly handle future users?
>
> That being said, I plan to merge HK_FLAG_RCU, HK_FLAG_MISC, HK_FLAG_SCHED,
> HK_FLAG_WQ and HK_FLAG_TIMER into HK_FLAG_UNBOUND since it doesn't make sense
> to divide them all.

That would be nice.

>  And the actual flag used inside cpumask_local_spread()
> could end up being HK_FLAG_DOMAIN | HK_FLAG_UNBOUND. So probably you don't
> need to worry about that and just change the HK_FLAG_WQ in your patch
> with HK_FLAG_MANAGED_IRQ.
>
> Thanks.
>
-- 
Thanks
Nitesh


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