On Mon, Jan 20, 2014 at 12:34:15PM +0100, Peter Zijlstra wrote: > On Mon, Jan 20, 2014 at 03:44:44AM +0000, Nicholas A. Bellinger wrote: > > From: Kent Overstreet <kmo@xxxxxxxxxxxxx> > > > > This patch changes percpu_ida_alloc() + callers to accept task state > > bitmask for prepare_to_wait() for code like target/iscsi that needs > > it for interruptible sleep, that is provided in a subsequent patch. > > > > It now expects TASK_UNINTERRUPTIBLE when the caller is able to sleep > > waiting for a new tag, or TASK_RUNNING when the caller cannot sleep, > > and is forced to return a negative value when no tags are available. > > > > v2 changes: > > - Include blk-mq + tcm_fc + vhost/scsi + target/iscsi changes > > - Drop signal_pending_state() call > > Urgh, you made me look at percpu_ida... steal_tags() does a > for_each_cpus() with IRQs disabled. This mean you'll disable IRQs for > multiple ticks on SGI class hardware. That is a _very_ long time indeed. It's not that bad in practice - the looping is limited by the number of other CPUs that actually have tags on their freelists - i.e. the CPUs that have recently been using that block device or whatever the percpu_ida is for. And we loop while cpu_have_tags is greater than some threshold (there's another debate about that) - the intention is not to steal tags unless too many other CPUs have tags on their local freelists. That said, for huge SGI class hardware I think you'd want the freelists to not be percpu, but rather be per core or something - that's probably a reasonable optimization for most hardware anyways. > Then there's alloc_global_tags() vs alloc_local_tags(), one gets an > actual tag, while the other only moves tags about -- semantic mismatch. Yeah, kind of. It is doing allocation, but not the same sort of allocation. > I do not get the comment near prepare to wait -- why does it matter if > percpu_ida_free() flips a cpus_have_tags bit? Did I write that comment? It is a crappy comment... Ok, in userspace we'd be using condition variables here, but this is the kernel so we need to carefully order putting ourselves on a waitlist, and checking the condition that determines whether we wait, and on the wakeup end changing things that affect that condition and doing the wakeup. steal_tags() is checking the condition that goes with the prepare_to_wait(), that's all. -- To unsubscribe from this list: send the line "unsubscribe target-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
