Hello, On Thu, Mar 23, 2023 at 04:00:31AM +0000, Yosry Ahmed wrote: > Currently, when sleeping is not allowed during rstat flushing, we hold > the global rstat lock with interrupts disabled throughout the entire > flush operation. Flushing in an O(# cgroups * # cpus) operation, and > having interrupts disabled throughout is dangerous. > > For some contexts, we may not want to sleep, but can be interrupted > (e.g. while holding a spinlock or RCU read lock). As such, do not > disable interrupts throughout rstat flushing, only when holding the > percpu lock. This breaks down the O(# cgroups * # cpus) duration with > interrupts disabled to a series of O(# cgroups) durations. > > Furthermore, if a cpu spinning waiting for the global rstat lock, it > doesn't need to spin with interrupts disabled anymore. I'm generally not a fan of big spin locks w/o irq protection. They too often become a source of unpredictable latency spikes. As you said, the global rstat lock can be held for quite a while. Removing _irq makes irq latency better on the CPU but on the other hand it makes a lot more likely that the lock is gonna be held even longer, possibly significantly so depending on the configuration and workload which will in turn stall other CPUs waiting for the lock. Sure, irqs are being serviced quicker but if the cost is more and longer !irq context multi-cpu stalls, what's the point? I don't think there's anything which requires the global lock to be held throughout the entire flushing sequence and irq needs to be disabled when grabbing the percpu lock anyway, so why not just release the global lock on CPU boundaries instead? We don't really lose anything significant that way. The durations of irq disabled sections are still about the same as in the currently proposed solution at O(# cgroups) and we avoid the risk of holding the global lock for too long unexpectedly from getting hit repeatedly by irqs while holding the global lock. Thanks. -- tejun
