Messages in this thread | | | Date | Tue, 12 Dec 2017 10:02:58 -0800 | From | Tejun Heo <> | Subject | Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU and generation based scheme |
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Hello, Peter.
On Tue, Dec 12, 2017 at 11:09:35AM +0100, Peter Zijlstra wrote: > > + /* > > + * ->aborted_gstate is used by the timeout to claim a specific > > + * recycle instance of this request. See blk_mq_timeout_work(). > > + */ > > + struct u64_stats_sync aborted_gstate_sync; > > + u64 aborted_gstate; > > So I dislike that u64_stats_sync thingy. Esp when used on a single > variable like this.
Hmm... I see.
> There are two alternatives, but I don't understand the code well enough > to judge the trade-offs. > > 1) use gstate_seq for this too; yes it will add some superfluous > instructions on 64bit targets, but if timeouts are a slow path > this might not matter.
For aborted_gstate, the heavier reader side is the completion hot path. That's two rmbs, which in itself isn't too much but is still difficult to justify.
> 2) use the pattern we use for cfs_rq::min_vruntime; namely: > > u64 aborted_gstate > #ifdef CONFIG_64BIT > u64 aborted_gstate_copy; > #endif > > > static inline void blk_mq_rq_set_abort(struct rq *rq, u64 gstate) > { > rq->aborted_gstate = gstate; > #ifdef CONFIG_64BIT > smp_wmb(); > rq->aborted_gstate_copy = gstate; > #endif > } > > static inline u64 blk_mq_rq_get_abort(struct rq *rq) > { > #ifdef CONFIG_64BIT > u64 abort, copy; > > do { > copy = rq->aborted_gstate_copy; > smp_rmb(); > abort = rq->aborted_gstate; > } while (abort != copy); > > return abort; > #else > return rq->aborted_gstate; > #endif > } > > which is actually _faster_ than the u64_stats_sync stuff (for a > single variable).
Hmm... doing the seq reading on the variable content itself, so if we had something like this as library, I'd be happy to use it but I really don't want to open-code this.
> But it might not matter; I just dislike that thing, could be me.
I'll leave it as-is for now. Probably the right thing to do in the longer term is adding the seq-reading-by-content-thing in the library.
Thanks.
-- tejun
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