Re: [PATCH v1 0/3] Avoid scheduling cache draining to isolated cpus

[Leonardo Brás]

On Thu, 2022-11-03 at 16:31 +0100, Michal Hocko wrote:
> On Thu 03-11-22 11:59:20, Leonardo Brás wrote:
> > On Wed, 2022-11-02 at 09:53 +0100, Michal Hocko wrote:
> > > On Tue 01-11-22 23:02:40, Leonardo Bras wrote:
> > > > Patch #1 expands housekeepíng_any_cpu() so we can find housekeeping cpus
> > > > closer (NUMA) to any desired CPU, instead of only the current CPU.
> > > > 
> > > > ### Performance argument that motivated the change:
> > > > There could be an argument of why would that be needed, since the current
> > > > CPU is probably acessing the current cacheline, and so having a CPU closer
> > > > to the current one is always the best choice since the cache invalidation
> > > > will take less time. OTOH, there could be cases like this which uses
> > > > perCPU variables, and we can have up to 3 different CPUs touching the
> > > > cacheline:
> > > > 
> > > > C1 - Isolated CPU: The perCPU data 'belongs' to this one
> > > > C2 - Scheduling CPU: Schedule some work to be done elsewhere, current cpu
> > > > C3 - Housekeeping CPU: This one will do the work
> > > > 
> > > > Most of the times the cacheline is touched, it should be by C1. Some times
> > > > a C2 will schedule work to run on C3, since C1 is isolated.
> > > > 
> > > > If C1 and C2 are in different NUMA nodes, we could have C3 either in
> > > > C2 NUMA node (housekeeping_any_cpu()) or in C1 NUMA node 
> > > > (housekeeping_any_cpu_from(C1). 
> > > > 
> > > > If C3 is in C2 NUMA node, there will be a faster invalidation when C3
> > > > tries to get cacheline exclusivity, and then a slower invalidation when
> > > > this happens in C1, when it's working in its data.
> > > > 
> > > > If C3 is in C1 NUMA node, there will be a slower invalidation when C3
> > > > tries to get cacheline exclusivity, and then a faster invalidation when
> > > > this happens in C1.
> > > > 
> > > > The thing is: it should be better to wait less when doing kernel work
> > > > on an isolated CPU, even at the cost of some housekeeping CPU waiting
> > > > a few more cycles.
> > > > ###
> > > > 
> > > > Patch #2 changes the locking strategy of memcg_stock_pcp->stock_lock from
> > > > local_lock to spinlocks, so it can be later used to do remote percpu
> > > > cache draining on patch #3. Most performance concerns should be pointed
> > > > in the commit log.
> > > > 
> > > > Patch #3 implements the remote per-CPU cache drain, making use of both 
> > > > patches #2 and #3. Performance-wise, in non-isolated scenarios, it should
> > > > introduce an extra function call and a single test to check if the CPU is
> > > > isolated. 
> > > > 
> > > > On scenarios with isolation enabled on boot, it will also introduce an
> > > > extra test to check in the cpumask if the CPU is isolated. If it is,
> > > > there will also be an extra read of the cpumask to look for a
> > > > housekeeping CPU.
> > > 
> > 
> > Hello Michael, thanks for reviewing!
> > 
> > > This is a rather deep dive in the cache line usage but the most
> > > important thing is really missing. Why do we want this change? From the
> > > context it seems that this is an actual fix for isolcpu= setup when
> > > remote (aka non isolated activity) interferes with isolated cpus by
> > > scheduling pcp charge caches on those cpus.
> > > 
> > > Is this understanding correct?
> > 
> > That's correct! The idea is to avoid scheduling work to isolated CPUs.
> > 
> > > If yes, how big of a problem that is?
> > 
> > The use case I have been following requires both isolcpus= and PREEMPT_RT, since
> > the isolated CPUs will be running a real-time workload. In this scenario,
> > getting any work done instead of the real-time workload may cause the system to
> > miss a deadline, which can be bad. 
> 
> OK, I see. But is memcg charging actually a RT friendly operation in the
> first place? Please note that this path can trigger memory reclaim and
> that is when any RT expectations are simply going down the drain.

I understand the spent time for charging is unpredictable as you said, since a
lot of slow stuff may or may not happen. 

> 
> > >  If you want a remote draining then
> > > you need some sort of locking (currently we rely on local lock). How
> > > come this locking is not going to cause a different form of disturbance?
> > 
> > If I did everything right, most of the extra work should be done either in non-
> > isolated (housekeeping) CPUs, or during a syscall. I mean, the pcp charge caches
> > will be happening on a housekeeping CPU, and the locking cost should be paid
> > there as we want to avoid doing that in the isolated CPUs. 

Sorry, I think this caused a misunderstanding: I meant "the pcp charge cache
drain will be happening on a housekeeping CPU, ..."

> > 
> > I understand there will be a locking cost being paid in the isolated CPUs when:
> > a) The isolated CPU is requesting the stock drain,
> > b) When the isolated CPUs do a syscall and end up using the protected structure
> > the first time after a remote drain.
> 
> And anytime the charging path (consume_stock resp. refill_stock)
> contends with the remote draining which is out of control of the RT
> task. It is true that the RT kernel will turn that spin lock into a
> sleeping RT lock and that could help with potential priority inversions
> but still quite costly thing I would expect.
> 
> > Both (a) and (b) should happen during a syscall, and IIUC the a rt workload
> > should not expect the syscalls to be have a predictable time, so it should be
> > fine.
> 
> Now I am not sure I understand. If you do not consider charging path to
> be RT sensitive then why is this needed in the first place? What else
> would be populating the pcp cache on the isolated cpu? IRQs?

I am mostly trying to deal with drain_all_stock() calling schedule_work_on() at
isolated_cpus. Since the scheduled drain_local_stock() will be competing for cpu
time with the RT workload, we can have preemption of the RT workload, which is a
problem for meeting the deadlines.

One way I thought to solve that was introducing a remote drain, which would
require a different strategy for locking, since not all accesses to the pcp
caches would happen on a local CPU. 

Then I tried to weight the costs of this, so the solution would introduce as
little overhead as possible on no-isolation scenarios. Also, for isolation
scenarios, I tried to put most of the overheads into the housekeeping CPUs, and
the remaining on the syscalls, which are also expected to be non-predictable.

Not sure if I could answer your question, though. Please let me know in case I
missed anything.

Thanks for helping me make it more clear!
Best regards,
Leo