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    SubjectRe: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free
    On Wed, Jan 18, 2023 at 11:01:08AM -0800, Suren Baghdasaryan wrote:
    > On Wed, Jan 18, 2023 at 10:34 AM Paul E. McKenney <paulmck@kernel.org> wrote:
    > >
    > > 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@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@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.
    >
    > 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.

    Whew!!! ;-)

    > Thanks for clarifications, Paul!

    No problem!

    Thanx, Paul

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