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

[Paul E. McKenney]

On Wed, Jan 18, 2023 at 10:04:39AM -0800, Suren Baghdasaryan wrote:
> On Wed, Jan 18, 2023 at 1:49 AM Michal Hocko <mhocko at suse.com> wrote:
> >
> > On Tue 17-01-23 17:19:46, Suren Baghdasaryan wrote:
> > > On Tue, Jan 17, 2023 at 7:57 AM Michal Hocko <mhocko at suse.com> wrote:
> > > >
> > > > On Mon 09-01-23 12:53:34, Suren Baghdasaryan wrote:
> > > > > call_rcu() can take a long time when callback offloading is enabled.
> > > > > Its use in the vm_area_free can cause regressions in the exit path when
> > > > > multiple VMAs are being freed.
> > > >
> > > > What kind of regressions.
> > > >
> > > > > To minimize that impact, place VMAs into
> > > > > a list and free them in groups using one call_rcu() call per group.
> > > >
> > > > Please add some data to justify this additional complexity.
> > >
> > > Sorry, should have done that in the first place. A 4.3% regression was
> > > noticed when running execl test from unixbench suite. spawn test also
> > > showed 1.6% regression. Profiling revealed that vma freeing was taking
> > > longer due to call_rcu() which is slow when RCU callback offloading is
> > > enabled.
> >
> > Could you be more specific? vma freeing is async with the RCU so how
> > come this has resulted in a regression? Is there any heavy
> > rcu_synchronize in the exec path? That would be an interesting
> > information.
> 
> No, there is no heavy rcu_synchronize() or any other additional
> synchronous load in the exit path. It's the call_rcu() which can block
> the caller if CONFIG_RCU_NOCB_CPU is enabled and there are lots of
> other call_rcu()'s going on in parallel. Note that call_rcu() calls
> rcu_nocb_try_bypass() if CONFIG_RCU_NOCB_CPU is enabled and profiling
> revealed that this function was taking multiple ms (don't recall the
> actual number, sorry). Paul's explanation implied that this happens
> due to contention on the locks taken in this function. For more
> in-depth details I'll have to ask Paul for help :) This code is quite
> complex and I don't know all the details of RCU implementation.

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.

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.

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.

Or have I lost track of some other slow case?

							Thanx, Paul