Messages in this thread | | | Date | Sat, 24 Jan 2009 16:52:15 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v3] softlockup: remove hung_task_check_count |
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On Fri, Jan 23, 2009 at 05:55:14PM -0800, Mandeep Singh Baines wrote: > Frédéric Weisbecker (fweisbec@gmail.com) wrote: > > 2009/1/23 Ingo Molnar <mingo@elte.hu>: > > > > > > not sure i like the whole idea of removing the max iterations check. In > > > theory if there's a _ton_ of tasks, we could spend a lot of time looping > > > there. So it always looked prudent to limit it somewhat. > > > > > > > Which means we can loose several of them. Would it hurt to iterate as > > much as possible along the task list, > > keeping some care about writers starvation and latency? > > BTW I thought about the slow work framework, but I can't retrieve > > it.... But this thread has already a slow priority. > > > > Would it be interesting to provide a way for rwlocks to know if there > > is writer waiting for the lock? > > Would be cool if that API existed. You could release the CPU and/or lock as > soon as either was contended for. You'd have the benefits of fine-grained > locking without the overhead of locking and unlocking multiple time. > > Currently, there is no bit that can tell you there is a writer waiting. You'd > probably need to change the write_lock() implementation at a minimum. Maybe > if the first writer left the RW_LOCK_BIAS bit clear and then waited for the > readers to leave instead of re-trying? That would actually make write_lock() > more efficient for the 1-writer case since you'd only need to spin doing > a read in the failure case instead of an atomic_dec and atomic_inc. >
This is already what is done in the slow path (in x86):
/* rdi: pointer to rwlock_t */ ENTRY(__write_lock_failed) CFI_STARTPROC LOCK_PREFIX addl $RW_LOCK_BIAS,(%rdi) 1: rep nop cmpl $RW_LOCK_BIAS,(%rdi) jne 1b LOCK_PREFIX subl $RW_LOCK_BIAS,(%rdi) jnz __write_lock_failed ret CFI_ENDPROC END(__write_lock_failed)
It spins lurking at the lock value and only if there are no writers nor readers that own the lock, it restarts its atomic_sub (and then atomic_add in fail case).
And if an implementation of writers_waiting_for_lock() is needed, I guess this is the perfect place. One atomic_add on a "waiters_count" on entry and an atomic_sub on it on exit.
Since this is the slow_path, I guess that wouldn't really impact the performances....
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