On Thu, Jan 19, 2023 at 01:52:14PM +0100, Michal Hocko wrote: > 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. If I am following along correctly (ha!), then your "induce a higher overhead" should be something like "induce a higher to-kfree() latency". Of course, there already is a higher latency-to-kfree via call_rcu() than via a direct call to kfree(), and callback-offload CPUs that are being flooded with callbacks raise that latency a jiffy or so more in order to avoid lock contention. If this becomes a problem, the callback-offloading code can be a bit smarter about avoiding lock contention, but need to see a real problem before I make that change. But if there is a real problem I will of course fix it. Or did I miss a turn in this discussion? Thanx, Paul
