On Wed, Jan 18, 2023 at 10:34 AM Paul E. McKenney <paulmck@xxxxxxxxxx> 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@xxxxxxxx> wrote: > > > > > > On Tue 17-01-23 17:19:46, Suren Baghdasaryan wrote: > > > > On Tue, Jan 17, 2023 at 7:57 AM Michal Hocko <mhocko@xxxxxxxx> 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. Thanks for clarifications, Paul! > > Or have I lost track of some other slow case? > > Thanx, Paul
