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. 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
