Messages in this thread | | | Date | Thu, 02 Feb 2006 02:31:48 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH] Avoid moving tasks when a schedule can be made. |
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Ingo Molnar wrote: > * Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>>If it were generated by some real workload that cares, then I would care. > > > well, you might not care, but i do. It's up to you what you care about, > but right now the scheduler policy is that we do care about latencies. > Yes, it's obviously all subject to common sense, and if something > triggers in a rare and extreme workload then any change related to it > has a _much_ higher barrier of acceptance than a common codepath. But > your blanket dismissal of this whole subject based on the rarity of the > workload is just plain wrong. >
No, if you read what I'd been saying, I'm not dismissing the whole subject based on the workload. I'm saying that there is no point to include such a "fix" based on the numbers given by this workload (if there is a more meaningful one, then sure). Especially not while there are sources of equivalent latency.
It is really simple: I can find a code path in the kernel, and work out how to exploit it by increasing resource loading until it goes bang (another example, tasklist_lock).
This is not really a justification for trying to "fix" it.
Unless somewhere there was an agreement that 1.5ms interrupt latency was a bug, full stop.
> >>>to argue that 'you can get the same by using rwsems so why should we >>>bother' is pretty lame: rwsems are rare and arguably broken in >>>behavior, and i'd not say the same about the scheduler (just yet :-). >> >>I don't think it is lame at all. They're fairly important in use in >>mmap_sem that I know of. And I have seen workloads where the up_write >>path gets really expensive (arguably more relevant ones than >>hackbench). > > > they are broken e.g. in that they are mass-waking all the readers with > interrupts disabled. At a minimum rwsems should be declared irq-unsafe > (like mutexes), as all the substantial uses are in process-context > codepaths anyway. I'll revisit rwsems once the current mutex work is > done. >
That would be great. Actually I have some patches that move the actual waking of the tasks out from underneath the lock too which gave some scalability benefits (and I'd imagine far less interrupt-off time, so let me know when you start work on rwsems).
But there are still places where interrupts can be held off for indefinite periods. I don't see why the scheduler lock is suddenly important - I could have told you years ago what would happen if you trigger the load balancer with enough tasks.
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