Re: [PATCH] Avoid moving tasks when a schedule can be made.

[Nick Piggin <nickpiggin@yahoo.com.au>]

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.

-- 
SUSE Labs, Novell Inc.
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