Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free

[Michal Hocko]

On Wed 18-01-23 11:01:08, Suren Baghdasaryan wrote:
> On Wed, Jan 18, 2023 at 10:34 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
[...]
> > There are a couple of possibilities here.
> >
> > First, if I am remembering correctly, the time between the call_rcu()
> > and invocation of the corresponding callback was taking multiple seconds,
> > but that was because the kernel was built with CONFIG_LAZY_RCU=y in
> > order to save power by batching RCU work over multiple call_rcu()
> > invocations.  If this is causing a problem for a given call site, the
> > shiny new call_rcu_hurry() can be used instead.  Doing this gets back
> > to the old-school non-laziness, but can of course consume more power.
> 
> That would not be the case because CONFIG_LAZY_RCU was not an option
> at the time I was profiling this issue.
> Laxy RCU would be a great option to replace this patch but
> unfortunately it's not the default behavior, so I would still have to
> implement this batching in case lazy RCU is not enabled.
> 
> >
> > Second, there is a much shorter one-jiffy delay between the call_rcu()
> > and the invocation of the corresponding callback in kernels built with
> > either CONFIG_NO_HZ_FULL=y (but only on CPUs mentioned in the nohz_full
> > or rcu_nocbs kernel boot parameters) or CONFIG_RCU_NOCB_CPU=y (but only
> > on CPUs mentioned in the rcu_nocbs kernel boot parameters).  The purpose
> > of this delay is to avoid lock contention, and so this delay is incurred
> > only on CPUs that are queuing callbacks at a rate exceeding 16K/second.
> > This is reduced to a per-jiffy limit, so on a HZ=1000 system, a CPU
> > invoking call_rcu() at least 16 times within a given jiffy will incur
> > the added delay.  The reason for this delay is the use of a separate
> > ->nocb_bypass list.  As Suren says, this bypass list is used to reduce
> > lock contention on the main ->cblist.  This is not needed in old-school
> > kernels built without either CONFIG_NO_HZ_FULL=y or CONFIG_RCU_NOCB_CPU=y
> > (including most datacenter kernels) because in that case the callbacks
> > enqueued by call_rcu() are touched only by the corresponding CPU, so
> > that there is no need for locks.
> 
> I believe this is the reason in my profiled case.
> 
> >
> > Third, if you are instead seeing multiple milliseconds of CPU consumed by
> > call_rcu() in the common case (for example, without the aid of interrupts,
> > NMIs, or SMIs), please do let me know.  That sounds to me like a bug.
> 
> I don't think I've seen such a case.
> Thanks for clarifications, Paul!

Thanks for the explanation Paul. I have to say this has caught me as a
surprise. There are just not enough details about the benchmark to
understand what is going on but I find it rather surprising that
call_rcu can induce a higher overhead than the actual kmem_cache_free
which is the callback. My naive understanding has been that call_rcu is
really fast way to defer the execution to the RCU safe context to do the
final cleanup.
-- 
Michal Hocko
SUSE Labs