Date: Sat, 28 Feb 2015 07:44:31 -0800 From: Harrison Grundy <harrison.grundy@astrodoggroup.com> To: freebsd-arch@freebsd.org Subject: Re: Minor ULE changes and optimizations Message-ID: <54F1E25F.5040905@astrodoggroup.com> In-Reply-To: <1547642.s3cC06khRt@ralph.baldwin.cx> References: <54EF2C54.7030207@astrodoggroup.com> <2311645.BNIPBaFv2E@ralph.baldwin.cx> <54F0925F.30002@astrodoggroup.com> <1547642.s3cC06khRt@ralph.baldwin.cx>
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On 02/28/15 04:24, John Baldwin wrote: > On Friday, February 27, 2015 07:50:55 AM Harrison Grundy wrote: >> On 02/27/15 06:14, John Baldwin wrote: >>> On Thursday, February 26, 2015 06:23:16 AM Harrison Grundy >>> wrote: >>>> https://reviews.freebsd.org/D1969 This allows a >>>> non-migratable thread to pin itself to a CPU if it is already >>>> running on that CPU. >>>> >>>> I've been running these patches for the past week or so >>>> without issue. Any additional testing or comments would be >>>> greatly appreciated. >>> >>> Can you explain the reason / use case for this? This seems to >>> be allowing an API violation. sched_pin() was designed to be >>> a lower-level API than sched_bind(), so you wouldn't call >>> sched_bind() if you were already pinned. In addition, >>> sched_pin() is sometimes used by code that assumes it won't >>> migrate until sched_unpin() (e.g. temporary mappings inside an >>> sfbuf). If you allow sched_bind() to move a thread that is >>> pinned you will allow someone to unintentionally break those >>> sort of things instead of getting an assertion failure panic. >> >> For a pinned thread, the underlying idea is that if you're >> already on the CPU you pinned to, calling sched_bind with that >> CPU specified allows you to set TSF_BOUND without calling >> sched_unpin first. >> >> If a pinned thread were to call sched_bind for a CPU it isn't >> pinned to, it would still hit the assert and fail. >> >> For any unpinned thread, if you're already running on the correct >> CPU, you can skip the THREAD_CAN_MIGRATE check and the call to >> mi_switch. > > Ah, ok, so you aren't allowing migration in theory. However, I'm > still curious as to why you want/need this. This makes the API > usage a bit more complex to reason about (sched_bind() can > sometimes be called while pinned but not always after this change), > so I think that extra complexity needs a reason to exist. Primarily, it allows those threads already on a CPU to skip the call to mi_switch and get out of sched_bind a bit faster. Additionally, it allows a driver to call sched_pin, then bind to that same cpu later without having to write something like "critical_enter(); sched_unpin(); sched_bind(foo, bar); critical_exit();", since otherwise it could be migrated/preempted between unpin and bind.
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