Message ID | 20240117163511.88173-1-longman@redhat.com |
---|---|
Headers | show |
Series | cgroup/cpuset: Support RCU_NOCB on isolated partitions | expand |
On 2/6/24 07:56, Frederic Weisbecker wrote: > Le Wed, Jan 17, 2024 at 12:15:07PM -0500, Waiman Long a écrit : >> On 1/17/24 12:07, Tejun Heo wrote: >>> Hello, >>> >>> On Wed, Jan 17, 2024 at 11:35:03AM -0500, Waiman Long wrote: >>>> The first 2 patches are adopted from Federic with minor twists to fix >>>> merge conflicts and compilation issue. The rests are for implementing >>>> the new cpuset.cpus.isolation_full interface which is essentially a flag >>>> to globally enable or disable full CPU isolation on isolated partitions. >>> I think the interface is a bit premature. The cpuset partition feature is >>> already pretty restrictive and makes it really clear that it's to isolate >>> the CPUs. I think it'd be better to just enable all the isolation features >>> by default. If there are valid use cases which can't be served without >>> disabling some isolation features, we can worry about adding the interface >>> at that point. >> My current thought is to make isolated partitions act like isolcpus=domain, >> additional CPU isolation capabilities are optional and can be turned on >> using isolation_full. However, I am fine with making all these turned on by >> default if it is the consensus. > Right it was the consensus last time I tried. Along with the fact that mutating > this isolation_full set has to be done on offline CPUs to simplify the whole > picture. > > So lemme try to summarize what needs to be done: > > 1) An all-isolation feature file (that is, all the HK_TYPE_* things) on/off for > now. And if it ever proves needed, provide a way later for more finegrained > tuning. That is more or less the current plan. As detailed below, HK_TYPE_DOMAIN & HK_TYPE_WQ isolation are included in the isolated partitions by default. I am also thinking about including other relatively cheap isolation flags by default. The expensive ones will have to be enabled via isolation_full. > > 2) This file must only apply to offline CPUs because it avoids migrations and > stuff. Well, the process of first moving the CPUs offline first is rather expensive. I won't mind doing some partial offlining based on the existing set of teardown and bringup callbacks, but I would try to avoid fully offlining the CPUs first. > > 3) I need to make RCU NOCB tunable only on offline CPUs, which isn't that much > changes. > > 4) HK_TYPE_TIMER: > * Wrt. timers in general, not much needs to be done, the CPUs are > offline. But: > * arch/x86/kvm/x86.c does something weird > * drivers/char/random.c might need some care > * watchdog needs to be (de-)activated > > 5) HK_TYPE_DOMAIN: > * This one I fear is not mutable, this is isolcpus... HK_TYPE_DOMAIN is already available via the current cpuset isolated partition functionality. What I am currently doing is to extend that to other HK_TYPE* flags. > > 6) HK_TYPE_MANAGED_IRQ: > * I prefer not to think about it :-) > > 7) HK_TYPE_TICK: > * Maybe some tiny ticks internals to revisit, I'll check that. > * There is a remote tick to take into consideration, but again the > CPUs are offline so it shouldn't be too complicated. > > 8) HK_TYPE_WQ: > * Fortunately we already have all the mutable interface in place. > But we must make it live nicely with the sysfs workqueue affinity > files. HK_TYPE_WQ is basically done and it is going to work properly with the workqueue affinity sysfs files. From the workqueue of view, HK_TYPE_WQ is currently treated the same as HK_TYPE_DOMAIN. > > 9) HK_FLAG_SCHED: > * Oops, this one is ignored by nohz_full/isolcpus, isn't it? > Should be removed? I don't think HK_FLAG_SCHED is being used at all. So I believe we should remove it to avoid confusion. > > 10) HK_TYPE_RCU: > * That's point 3) and also some kthreads to affine, which leads us > to the following in HK_TYPE_KTHREAD: > > 11) HK_FLAG_KTHREAD: > * I'm guessing it's fine as long as isolation_full is also an > isolated partition. Then unbound kthreads shouldn't run there. Yes, isolation_full applies only to isolated partitions. It extends the amount of CPU isolation by enabling all the other CPU available isolation flags. Cheers, Longman
On 1/19/24 05:24, Paul E. McKenney wrote: > On Wed, Jan 17, 2024 at 11:35:03AM -0500, Waiman Long wrote: >> This patch series is based on the RFC patch from Frederic [1]. Instead >> of offering RCU_NOCB as a separate option, it is now lumped into a >> root-only cpuset.cpus.isolation_full flag that will enable all the >> additional CPU isolation capabilities available for isolated partitions >> if set. RCU_NOCB is just the first one to this party. Additional dynamic >> CPU isolation capabilities will be added in the future. >> >> The first 2 patches are adopted from Federic with minor twists to fix >> merge conflicts and compilation issue. The rests are for implementing >> the new cpuset.cpus.isolation_full interface which is essentially a flag >> to globally enable or disable full CPU isolation on isolated partitions. >> On read, it also shows the CPU isolation capabilities that are currently >> enabled. RCU_NOCB requires that the rcu_nocbs option be present in >> the kernel boot command line. Without that, the rcu_nocb functionality >> cannot be enabled even if the isolation_full flag is set. So we allow >> users to check the isolation_full file to verify that if the desired >> CPU isolation capability is enabled or not. >> >> Only sanity checking has been done so far. More testing, especially on >> the RCU side, will be needed. > There has been some discussion of simplifying the (de-)offloading code > to handle only offline CPUs. Along with some discussion of eliminating > the (de-)offloading capability altogehter. > > We clearly should converge on the capability to be provided before > exposing this to userspace. ;-) Would you mind giving me a pointer to the discussion of simplifying the de-offloading code to handle only offline CPUs? Thanks, Longman