Messages in this thread | | | From | jun qian <> | Date | Wed, 29 Jul 2020 20:51:12 +0800 | Subject | Re: [PATCH V4] Softirq:avoid large sched delay from the pending softirqs |
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On Wed, Jul 29, 2020 at 8:16 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > Qian, > > jun qian <qianjun.kernel@gmail.com> writes: > > On Mon, Jul 27, 2020 at 11:41 PM Thomas Gleixner <tglx@linutronix.de> wrote: > >> > + or_softirq_pending(pending << (vec_nr + 1)); > >> > >> To or the value interrupts need to be disabled because otherwise you can > >> lose a bit when an interrupt happens in the middle of the RMW operation > >> and raises a softirq which is not in @pending and not in the per CPU > >> local softirq pending storage. > >> > > I can't understand the problem described above, could you explain in > > detail. > > Oring a value to a memory location is a Read Modify Write (RMW) > operation. > > x |= a; > > translates to pseudo code: > > R1 = x // Load content of memory location x into register R1 > R1 |= a // Or value a on R1 > x = R1 // Write back result > > So assume: > > x = 0 > a = 1 > > R1 = x --> R1 == 0 > R1 |= a --> R1 == 1 > > interrupt sets bit 1 in x --> x == 0x02 > > x = R1 --> x == 1 > > So you lost the set bit 1, right? Not really what you want. > wow, thanks a lot, i got it.
> >> There is another problem. Assume bit 0 and 1 are pending when the > >> processing starts. Now it breaks out after bit 0 has been handled and > >> stores back bit 1 as pending. Before ksoftirqd runs bit 0 gets raised > >> again. ksoftirqd runs and handles bit 0, which takes more than the > >> timeout. As a result the bit 0 processing can starve all other softirqs. > >> > > May I use a percpu val to record the order of processing softirq, by the order > > val, when it is in ksoftirqd we can process the pending softirq in order. In the > > scenario you described above, before ksoftirqd runs, bit 0 gets raised again, > > ksoftirqd runs and handles bit 1 by the percpu val. just like a ring. > > Yes, you need something to save information about the not-processed > bits. Keeping track of which bit to process next works, but whether > that's going to result in efficient and simple code is a different > question. > ok, i will modify it in the next version.
> Thanks, > > tglx >
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