Message-ID: <ec8088fa-0312-4e98-9e0e-ba9a60106d58@paulmck-laptop>
Date: Thu, 20 Jun 2024 10:26:38 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Leonardo Bras <leobras.c@...il.com>
Cc: Leonardo Bras <leobras@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] kvm: Note an RCU quiescent state on guest exit

On Thu, Jun 20, 2024 at 04:03:41AM -0300, Leonardo Bras wrote:
> On Wed, May 15, 2024 at 07:57:41AM -0700, Paul E. McKenney wrote:
> > On Wed, May 15, 2024 at 01:45:33AM -0300, Leonardo Bras wrote:
> > > On Tue, May 14, 2024 at 03:54:16PM -0700, Paul E. McKenney wrote:
> > > > On Mon, May 13, 2024 at 06:47:13PM -0300, Leonardo Bras Soares Passos wrote:
> > > > > On Mon, May 13, 2024 at 4:40 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > > > >
> > > > > > On Fri, May 10, 2024, Leonardo Bras wrote:
> > > > > > > As of today, KVM notes a quiescent state only in guest entry, which is good
> > > > > > > as it avoids the guest being interrupted for current RCU operations.
> > > > > > >
> > > > > > > While the guest vcpu runs, it can be interrupted by a timer IRQ that will
> > > > > > > check for any RCU operations waiting for this CPU. In case there are any of
> > > > > > > such, it invokes rcu_core() in order to sched-out the current thread and
> > > > > > > note a quiescent state.
> > > > > > >
> > > > > > > This occasional schedule work will introduce tens of microsseconds of
> > > > > > > latency, which is really bad for vcpus running latency-sensitive
> > > > > > > applications, such as real-time workloads.
> > > > > > >
> > > > > > > So, note a quiescent state in guest exit, so the interrupted guests is able
> > > > > > > to deal with any pending RCU operations before being required to invoke
> > > > > > > rcu_core(), and thus avoid the overhead of related scheduler work.
> > > > > >
> > > > > > Are there any downsides to this?  E.g. extra latency or anything?  KVM will note
> > > > > > a context switch on the next VM-Enter, so even if there is extra latency or
> > > > > > something, KVM will eventually take the hit in the common case no matter what.
> > > > > > But I know some setups are sensitive to handling select VM-Exits as soon as possible.
> > > > > >
> > > > > > I ask mainly because it seems like a no brainer to me to have both VM-Entry and
> > > > > > VM-Exit note the context switch, which begs the question of why KVM isn't already
> > > > > > doing that.  I assume it was just oversight when commit 126a6a542446 ("kvm,rcu,nohz:
> > > > > > use RCU extended quiescent state when running KVM guest") handled the VM-Entry
> > > > > > case?
> > > > > 
> > > > > I don't know, by the lore I see it happening in guest entry since the
> > > > > first time it was introduced at
> > > > > https://lore.kernel.org/all/1423167832-17609-5-git-send-email-riel@redhat.com/
> > > > > 
> > > > > Noting a quiescent state is cheap, but it may cost a few accesses to
> > > > > possibly non-local cachelines. (Not an expert in this, Paul please let
> > > > > me know if I got it wrong).
> > > > 
> > > > Yes, it is cheap, especially if interrupts are already disabled.
> > > > (As in the scheduler asks RCU to do the same amount of work on its
> > > > context-switch fastpath.)
> > > 
> > > Thanks!
> > > 
> > > > 
> > > > > I don't have a historic context on why it was just implemented on
> > > > > guest_entry, but it would make sense when we don't worry about latency
> > > > > to take the entry-only approach:
> > > > > - It saves the overhead of calling rcu_virt_note_context_switch()
> > > > > twice per guest entry in the loop
> > > > > - KVM will probably run guest entry soon after guest exit (in loop),
> > > > > so there is no need to run it twice
> > > > > - Eventually running rcu_core() may be cheaper than noting quiescent
> > > > > state every guest entry/exit cycle
> > > > > 
> > > > > Upsides of the new strategy:
> > > > > - Noting a quiescent state in guest exit avoids calling rcu_core() if
> > > > > there was a grace period request while guest was running, and timer
> > > > > interrupt hits the cpu.
> > > > > - If the loop re-enter quickly there is a high chance that guest
> > > > > entry's rcu_virt_note_context_switch() will be fast (local cacheline)
> > > > > as there is low probability of a grace period request happening
> > > > > between exit & re-entry.
> > > > > - It allows us to use the rcu patience strategy to avoid rcu_core()
> > > > > running if any grace period request happens between guest exit and
> > > > > guest re-entry, which is very important for low latency workloads
> > > > > running on guests as it reduces maximum latency in long runs.
> > > > > 
> > > > > What do you think?
> > > > 
> > > > Try both on the workload of interest with appropriate tracing and
> > > > see what happens?  The hardware's opinion overrides mine.  ;-)
> > > 
> > > That's a great approach!
> > > 
> > > But in this case I think noting a quiescent state in guest exit is 
> > > necessary to avoid a scenario in which a VM takes longer than RCU 
> > > patience, and it ends up running rcuc in a nohz_full cpu, even if guest 
> > > exit was quite brief. 
> > > 
> > > IIUC Sean's question is more on the tone of "Why KVM does not note a 
> > > quiescent state in guest exit already, if it does in guest entry", and I 
> > > just came with a few arguments to try finding a possible rationale, since 
> > > I could find no discussion on that topic in the lore for the original 
> > > commit.
> > 
> > Understood, and maybe trying it would answer that question quickly.
> > Don't get me wrong, just because it appears to work in a few tests doesn't
> > mean that it really works, but if it visibly blows up, that answers the
> > question quite quickly and easily.  ;-)
> > 
> > But yes, if it appears to work, there must be a full investigation into
> > whether or not the change really is safe.
> > 
> > 							Thanx, Paul
> 
> Hello Paul, Sean, sorry for the delay on this.
> 
> I tested x86 by counting cycles (using rdtsc_ordered()).
> 
> Cycles were counted upon function entry/exit on 
> {svm,vmx}_vcpu_enter_exit(), and right before / after 
> __{svm,vmx}_vcpu_run() in the same function.
> 
> The main idea was to get cycles spend in the procedures before entering 
> guest (such as reporting RCU quiescent state in entry / exit) and the 
> cycles actually used by the VM. 
> 
> Those cycles were summed-up and stored in per-cpu structures, with a 
> counter to get the average value. I then created a debug file to read the 
> results and reset the counters.
> 
> As for the VM, it got 20 vcpus, 8GB memory, and was booted with idle=poll.
> 
> The workload inside the VM consisted in cyclictest in 16 vcpus 
> (SCHED_FIFO,p95), while maintaining it's main routine in 4 other cpus 
> (SCHED_OTHER). This was made to somehow simulate busy and idle-er cpus. 
> 
>  $cyclictest -m -q -p95 --policy=fifo -D 1h -h60 -t 16 -a 4-19 -i 200 
>   --mainaffinity 0-3
> 
> All tests were run for exaclty 1 hour, and the clock counter was reset at 
> the same moment cyclictest stared. After that VM was poweroff from guest.
> Results show the average for all CPUs in the same category, in cycles.
> 
> With above setup, I tested 2 use cases:
> 1 - Non-RT host, no CPU Isolation, no RCU patience (regular use-case)
> 2 - PREEMPT_RT host, with CPU Isolation for all vcpus (pinned), and 
>     RCU patience = 1000ms (best case for RT)
> 
> Results are:
> # Test case 1:
> Vanilla: (average on all vcpus)
> VM Cycles / RT vcpu:		123287.75 
> VM Cycles / non-RT vcpu:	709847.25
> Setup Cycles:			186.00
> VM entries / RT vcpu:		58737094.81
> VM entries / non-RT vcpu:	10527869.25
> Total cycles in RT VM:		7241564260969.80
> Total cycles in non-RT VM:	7473179035472.06
> 
> Patched: (average on all vcpus)
> VM Cycles / RT vcpu:		124695.31        (+ 1.14%)
> VM Cycles / non-RT vcpu:	710479.00        (+ 0.09%)
> Setup Cycles:			218.65           (+17.55%)
> VM entries / RT vcpu:		60654285.44      (+ 3.26%) 
> VM entries / non-RT vcpu:	11003516.75      (+ 4.52%)
> Total cycles in RT VM:		7563305077093.26 (+ 4.44%)
> Total cycles in non-RT VM:	7817767577023.25 (+ 4.61%)
> 
> Discussion:
> Setup cycles raised in ~33 cycles, increasing overhead.
> It proves that noting a quiescent state in guest entry introduces setup 
> routine costs, which is expected.
> 
> On the other hand, both the average time spend inside the VM and the number 
> of VM entries raised, causing the VM to have ~4.5% more cpu cycles 
> available to run, which is positive. Extra cycles probably came from not 
> having invoke_rcu_core() getting ran after VM exit.
> 
> 
> # Test case 2:
> Vanilla: (average on all vcpus)
> VM Cycles / RT vcpu:		123785.63
> VM Cycles / non-RT vcpu:	698758.25
> Setup Cycles:			187.20
> VM entries / RT vcpu:		61096820.75
> VM entries / non-RT vcpu:	11191873.00
> Total cycles in RT VM:		7562908142051.72
> Total cycles in non-RT VM:	7820413591702.25
> 
> Patched: (average on all vcpus)
> VM Cycles / RT vcpu:		123137.13        (- 0.52%)
> VM Cycles / non-RT vcpu:	696824.25        (- 0.28%)
> Setup Cycles:			229.35           (+22.52%)
> VM entries / RT vcpu:		61424897.13      (+ 0.54%) 
> VM entries / non-RT vcpu:	11237660.50      (+ 0.41%)
> Total cycles in RT VM:		7563685235393.27 (+ 0.01%)
> Total cycles in non-RT VM:	7830674349667.13 (+ 0.13%)
> 
> Discussion:
> Setup cycles raised in ~42 cycles, increasing overhead.
> It proves that noting a quiescent state in guest entry introduces setup 
> routine costs, which is expected.
> 
> The average time spend inside the VM was reduced, but the number of VM  
> entries raised, causing the VM to have around the same number of cpu cycles 
> available to run, meaning that the overhead caused by reporting RCU 
> quiescent state in VM exit got absorbed, and it may have to do with those 
> rare invoke_rcu_core()s that were bothering latency.
> 
> The difference is much smaller compared to case 1, and this is probably 
> because there is a clause in rcu_pending() for isolated (nohz_full) cpus 
> which may be already inhibiting a lot of invoke_rcu_core()s.
> 
> Sean, Paul, what do you think?

First, thank you for doing this work!

I must defer to Sean on how to handle this tradeoff.  My kneejerk reaction
would be to add yet another knob, but knobs are not free.

							Thanx, Paul

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