Messages in this thread | | | Date | Fri, 1 Oct 2021 16:27:43 -0700 | Subject | Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem() | From | John Hubbard <> |
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On 10/1/21 15:46, Jason Gunthorpe wrote: > On Fri, Oct 01, 2021 at 04:22:28PM -0600, Logan Gunthorpe wrote: > >>> It would close this issue, however synchronize_rcu() is very slow >>> (think > 1second) in some cases and thus cannot be inserted here. >> >> It shouldn't be *that* slow, at least not the vast majority of the >> time... it seems a bit unreasonable that a CPU wouldn't schedule for >> more than a second. > > I've seen bug reports on exactly this, it is well known. Loaded > big multi-cpu systems have high delays here, for whatever reason. >
So have I. One reason is that synchronize_rcu() doesn't merely wait for a context switch on each CPU--it also waits for callbacks (such as those set up by call_rcu(), if I understand correctly) to run.
These can really add up to something quite substantial. In fact, I don't think there is an upper limit on the running times, anywhere.
thanks, -- John Hubbard NVIDIA
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