Messages in this thread | | | Date | Tue, 11 Sep 2018 16:20:24 +0800 | From | Ming Lei <> | Subject | Re: [PATCH] percpu-refcount: relax limit on percpu_ref_reinit() |
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On Tue, Sep 11, 2018 at 12:40:36PM +0800, jianchao.wang wrote: > Hi Ming > > On 09/11/2018 12:03 PM, Ming Lei wrote: > > After the refcount is switched to atomic mode, the whole counting is > > done on the atomic part. Then if the refcount need to switch to percpu mode > > again, all percpu part of the counter is re-initialized as zero simply. This > > is invariant with/without this patch. > > Does the "whole counting" here means ? > > (long)(sum (every cpu's refcounter)) + atomic refcounter
No.
We only check the 'whole counting' at atomic mode, so it is the atomic part('ref->count'), please see percpu_ref_put_many(), in which ref->release() is only called at atomic mode.
At percpu mode, the ref-count is only decreased/increased on the local CPU.
> > and when switch to atomic mode, there could be value left in atomic refcounter. > then the unsigned long percpu refcounter cound be decreased from 0. > > From another angle, one request could be ended on a different cpu from the one where > it is issued.
Right.
But,
In the fast path(either completion or issue path), the .q_usage_counter is only increased/decreased, which can be done in either atomic or percpu mode.
And when we want to check the queued/inflight requests, the .q_usage_counter has to be killed first by switching to atomic mode, such as in all kinds of uses of blk_freeze_queue_start().
Thanks, Ming
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