Messages in this thread | | | Date | Mon, 22 Oct 2018 13:42:00 +0530 | From | Pavan Kondeti <> | Subject | Re: [RFC PATCH 29/30] softirq: Make softirq processing softinterruptible |
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Hi Frederic,
On Wed, Oct 17, 2018 at 02:26:02AM +0200, Frederic Weisbecker wrote: > Hi Pavan, > > On Tue, Oct 16, 2018 at 09:45:52AM +0530, Pavan Kondeti wrote: > > Hi Frederic, > > > > On Thu, Oct 11, 2018 at 01:12:16AM +0200, Frederic Weisbecker wrote: > > > From: Frederic Weisbecker <fweisbec@gmail.com> > > > > > > Make do_softirq() re-entrant and allow a vector, being either processed > > > or disabled, to be interrupted by another vector. This way a vector > > > won't be able to monopolize the CPU for a long while at the expense of > > > the others that may rely on some predictable latency, especially on > > > softirq disabled sections that used to disable all vectors. > > > > > I understand that a long running softirq can be preempted/interrupted by > > other softirqs which is not possible today. I have few questions on your > > patches. > > > > (1) When softirq processing is pushed to ksoftirqd, then the long running > > softirq can still block other softirqs (not in SOFTIRQ_NOW_MASK) for a while. > > correct? > > No, Ksoftirqd is treated the same as IRQ tail processing here: a vector can > interrupt another. So for example, a NET_RX softirq running in Ksoftirqd can > be interrupted by a TIMER softirq running in hardirq tail. > When ksoftirqd is running, we are only allowing softirqs in SOFTIRQ_NOW_MASK to run after serving an interrupt. So I don't see how TIMER which is not in SOFTIRQ_NOW_MASK can interrupt a NET_RX softirq running in ksoftirqd context.
> > > > (2) When softirqs processing happens asynchronously, a particular softirq > > like TASKLET can keep interrupting an already running softirq like TIMER/NET_RX, > > correct? In worse case scenario, a long running softirq like NET_RX interrupt > > a TIMER softirq. But I guess this is something expected with this. i.e > > each softirq is independent and whichever comes recent gets to interrupt the > > previously running softirqs. > > Exactly, and that's inherent with interrupts in general. The only way to work > around that is to thread each vector independantly but that's a whole different > dimension :-) > Right.
Assigning a thread for each vector also may not solve this problem because preemption would be disabled while a softirq vector is running in its own thread.
I guess there is no hard priorities among softirq vectors. Earlier it was like first come first serve, now it is not. If we had priorities defined, (don't know how :-)) we could disable the lower prio vectors while a higher prio vector is being handled. This way we could gaurantee that TIMER softirq or HI-TASKLET won't be starved while a long running softirq like NET_RX/NET_TX/RCU is running.
Thanks, Pavan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
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