Re: [PATCH] psi: reduce min window size to 50ms

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Feb 10, 2023 at 5:46 PM Sudarshan Rajagopalan
<quic_sudaraja@quicinc.com> wrote:
>
>
> On 2/10/2023 5:09 PM, Suren Baghdasaryan wrote:
> > On Fri, Feb 10, 2023 at 4:45 PM Sudarshan Rajagopalan
> > <quic_sudaraja@quicinc.com> wrote:
> >>
> >> On 2/10/2023 3:03 PM, Suren Baghdasaryan wrote:
> >>> On Fri, Feb 10, 2023 at 2:31 PM Sudarshan Rajagopalan
> >>> <quic_sudaraja@quicinc.com> wrote:
> >>>> The PSI mechanism is useful tool to monitor pressure stall
> >>>> information in the system. Currently, the minimum window size
> >>>> is set to 500ms. May we know what is the rationale for this?
> >>> The limit was set to avoid regressions in performance and power
> >>> consumption if the window is set too small and the system ends up
> >>> polling too frequently. That said, the limit was chosen based on
> >>> results of specific experiments which might not represent all
> >> Rightly as you said, the effect on power and performance depends on type
> >> of the system - embedded systems, or Android mobile, or commercial VMs
> >> or servers. With higher PSI sampling, it may not be much of power impact
> >> to embedded systems with low-tier chipsets or performance impact to
> >> powerful servers.
> >>
> >>> usecases. If you want to change this limit, you would need to describe
> >>> why the new limit is inherently better than the current one (why not
> >>> higher, why not lower).
> >> This is in regards to the userspace daemon [1] that we are working on,
> >> that dynamically resizes the VM memory based on PSI memory pressure
> >> events. With current min window size of 500ms, the PSI monitor sampling
> >> period would be 50ms. So to detect increase in memory demand in system
> >> and plug-in memory into VM when pressure goes up, the minimum time the
> >> process needs to stall for is 50ms before a event can be generated and
> >> sent out to userspace and the daemon can do actions.
> >>
> >> This again I'm talking w.r.t. lightweight embedded systems, where even
> >> background kswapd/kcompd (which I'm calling it as natural memory
> >> pressure) in the system would be less than 5-10ms stall. So any stall
> >> more than 5-10ms would "hint" us that a memory consuming usecase has
> >> ranB  and memory may need to be plugged in.
> >>
> >> So in these cases, having as low as 5ms psimon sampling time would give
> >> us faster reaction time and daemon can be responsive more quickly. In
> >> general, this will reduce the malloc latencies significantly.
> >>
> >> Pasting here the same excerpt I mentioned in [1].
> > My question is: why do you think 5ms is the optimal limit here? I want
> > to avoid a race to the bottom where next time someone can argue that
> > they would like to detect a stall within a lower period than 5ms.
> > Technically the limit can be as small as one wants but at some point I
> > think we should consider the possibility of this being used for a DoS
> > attack.
>
> Well the optimal limit should be something which is least destructive? I
> do understand about possibility of DoS attacks, but wouldn't that still
> be possible with 500ms window today? Which will atleast be 1/10th less
> severe compared to 50ms window. The way I see it is - min pressure
> sampling should be such that even the least pressure stall which we
> think is significant should be captured (this could be 5ms or 50ms at
> present) while balancing the power and performance impact across all
> usecases.
>
> At present, Android's LMKD sets 1000ms as window for which it considers
> 100ms sampling to be significant. And here, with psi_daemon usecase we
> are saying 5ms sampling would be significant. So there's no actual
> optimal limit, but we must limit as much possible without effecting
> power or performance as a whole. Also, this is just the "minimum
> allowable" window, and system admins can configure it as per the system
> type/requirement.

Ok, let me ask you another way which might be more productive. What
caused you to choose 5ms as the time you care to react to a stall
buildup?

>
> Also, about possible DoS attacks - file permissions for
> /proc/pressure/... can be set such that not any random user can register
> to psi events right?

True. We have a CAP_SYS_RESOURCE check for the writers of these files.

>
> >
> >> "
> >>
> >> 4. Detecting increase in memory demand b   when a certain usecase starts
> >> in VM that does memory allocations, it will stall causing PSI mechanism
> >> to generate a memory pressure event to userspace. To simply put, when
> >> pressure increases certain set threshold, it can make educated guess
> >> that a memory requiring usecase has ran and VM system needs memory to be
> >> added.
> >>
> >> "
> >>
> >> [1]
> >> https://lore.kernel.org/linux-arm-kernel/1bf30145-22a5-cc46-e583-25053460b105@redhat.com/T/#m95ccf038c568271e759a277a08b8e44e51e8f90b
> >>
> >>> Thanks,
> >>> Suren.
> >>>
> >>>> For lightweight systems such as Linux Embedded Systems, PSI
> >>>> can be used to monitor and track memory pressure building up
> >>>> in the system and respond quickly to such memory demands.
> >>>> Example, the Linux Embedded Systems could be a secondary VM
> >>>> system which requests for memory from Primary host. With 500ms
> >>>> window size, the sampling period is 50ms (one-tenth of windwo
> >>>> size). So the minimum amount of time the process needs to stall,
> >>>> so that a PSI event can be generated and actions can be done
> >>>> is 50ms. This reaction time can be much reduced by reducing the
> >>>> sampling time (by reducing window size), so that responses to
> >>>> such memory pressures in system can be serviced much quicker.
> >>>>
> >>>> Please let us know your thoughts on reducing window size to 50ms.
> >>>>
> >>>> Sudarshan Rajagopalan (1):
> >>>>     psi: reduce min window size to 50ms
> >>>>
> >>>>    kernel/sched/psi.c | 2 +-
> >>>>    1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> --
> >>>> 2.7.4
> >>>>