Date: Sat, 28 Feb 2015 07:24:30 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: Harrison Grundy <harrison.grundy@astrodoggroup.com> Subject: Re: Minor ULE changes and optimizations Message-ID: <1547642.s3cC06khRt@ralph.baldwin.cx> In-Reply-To: <54F0925F.30002@astrodoggroup.com> References: <54EF2C54.7030207@astrodoggroup.com> <2311645.BNIPBaFv2E@ralph.baldwin.cx> <54F0925F.30002@astrodoggroup.com>
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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. -- John Baldwin
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