Re: [RFC PATCH v2 2/4] perf: Allow adding fixed random jitter to the alternate sampling period

[James Clark <james.clark@arm.com>]

On 22/04/2024 15:40, Ben Gainey wrote:
> On Mon, 2024-04-22 at 14:08 +0100, James Clark wrote:
>>
>>
>> On 22/04/2024 11:49, Ben Gainey wrote:
>>> This change modifies the core perf overflow handler, adding some
>>> small
>>> random jitter to each sample period whenever an event switches
>>> between the
>>> two alternate sample periods. A new flag is added to
>>> perf_event_attr to
>>> opt into this behaviour.
>>>
>>> This change follows the discussion in [1], where it is recognized
>>> that it
>>> may be possible for certain patterns of execution to end up with
>>> biased
>>> results.
>>>
>>> [1] https://lore.kernel.org/linux-perf-
>>> users/Zc24eLqZycmIg3d2@tassilo/
>>>
>>> Signed-off-by: Ben Gainey <ben.gainey@arm.com>
>>> ---
>>>  include/uapi/linux/perf_event.h |  3 ++-
>>>  kernel/events/core.c            | 11 ++++++++++-
>>>  2 files changed, 12 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/uapi/linux/perf_event.h
>>> b/include/uapi/linux/perf_event.h
>>> index 5c1701d091cf..dd3697a4b300 100644
>>> --- a/include/uapi/linux/perf_event.h
>>> +++ b/include/uapi/linux/perf_event.h
>>> @@ -461,7 +461,8 @@ struct perf_event_attr {
>>>   inherit_thread :  1, /* children only inherit if cloned with
>>> CLONE_THREAD */
>>>   remove_on_exec :  1, /* event is removed from task on exec */
>>>   sigtrap        :  1, /* send synchronous SIGTRAP on event */
>>> - __reserved_1   : 26;
>>> + jitter_alternate_period : 1, /* add a limited amount of jitter on
>>> each alternate period */
>>> + __reserved_1   : 25;
>>>
>>>   union {
>>>   __u32 wakeup_events;   /* wakeup every n events */
>>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>>> index 07f1f931e18e..079ae520e836 100644
>>> --- a/kernel/events/core.c
>>> +++ b/kernel/events/core.c
>>> @@ -15,6 +15,7 @@
>>>  #include <linux/idr.h>
>>>  #include <linux/file.h>
>>>  #include <linux/poll.h>
>>> +#include <linux/random.h>
>>>  #include <linux/slab.h>
>>>  #include <linux/hash.h>
>>>  #include <linux/tick.h>
>>> @@ -9546,6 +9547,8 @@ static inline bool sample_is_allowed(struct
>>> perf_event *event, struct pt_regs *r
>>>   return true;
>>>  }
>>>
>>> +# define MAX_ALT_SAMPLE_PERIOD_JITTER 16
>>> +
>>
>> Is 16 enough to make a difference with very large alternate periods?
>> I'm
>> wondering if it's worth making it customisable and instead of adding
>> the
>> boolean option add a 16 bit jitter field. Or the option could still
>> be a
>> boolean but the jitter value is some ratio of the alt sample period,
>> like:
>>
>>   get_random_u32_below(max(16, alternative_sample_period >> 4))
>>
> 
> I don't really have a strong opinion; in all my time I've never seen an
> Arm PMU produce a precise and constant period anyway, so this may be
> more useful in the case the architecture is able to support precise
> sampling. In any case it's is likely to be specific to a particular
> workload / configuration anyway.
> 
> The main downside I can see for making it configurable is that the
> compiler cannot then optimise the get_random call as well as for a
> constant, which may be undesirable on this code path.
> 
> 

Hmmm I see, I didn't expect get_random_u32_below() to have such
different implementations depending on whether it was a constant or not.
You don't have to remove the constant though, it could be:

 get_random_u32() & (jitter_power_of_2_max - 1)

If you're really worried about optimising this path, then generating the
jitter with some rotate/xor/mask operation is probably much faster. I
don't think the requirements are for it to actually be "random", but
just something to perturb it, even a badly distributed random number
would be fine.

But also yes I don't have a particularly strong opinion either. Just
that if someone does have a justification for a configurable value in
the future, depending on how that's implemented it could make the new
jitter boolean redundant which would be annoying.