Messages in this thread | | | Date | Thu, 7 May 2015 14:49:39 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry |
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* Ingo Molnar <mingo@kernel.org> wrote:
> The TIF_RCU_QS thing is just a fancy way for synchronize_rcu() > (being executed on some other CPU not doing RT work) to > intelligently wait for the remote (RT work doing) CPU to finish > executing kernel code, without polling or so.
it's basically a cheap IPI being inserted on the remote CPU.
We need the TIF_RCU_QS callback not just to wait intelligently, but mainly to elapse a grace period, otherwise synchronize_rcu() might not ever make progress: think a SCHED_FIFO task doing some kernel work, synchronize_rcu() stumbling upon it - but the SCHED_FIFO task otherwise never scheduling and never getting any timer irqs either, and thus never entering quiescent state.
(Cc:-ed Paul too, he might be interested in this as well.)
Thanks,
Ingo
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