Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

[Rik van Riel <riel@redhat.com>]

On 05/01/2015 02:40 PM, Ingo Molnar wrote:

> Or we could do that in the syscall path with a single store of a 
> constant flag to a location in the task struct. We have a number of 
> natural flags that get written on syscall entry, such as:
> 
>         pushq_cfi $__USER_DS                    /* pt_regs->ss */
> 
> That goes to a constant location on the kernel stack. On return from 
> system calls we could write 0 to that location.
> 
> So the remote CPU would have to do a read of this location. There are 
> two cases:
> 
>  - If it's 0, then it has observed quiescent state on that CPU. (It 
>    does not have to be atomics anymore, as we'd only observe the value 
>    and MESI coherency takes care of it.)

That should do the trick.

>  - If it's not 0 then the remote CPU is not executing user-space code 
>    and we can install (remotely) a TIF_NOHZ flag in it and expect it 
>    to process it either on return to user-space or on a context 
>    switch.

I may have to think about this a little more, but
it seems like it should work.

Can we use a separate byte in the flags word for
flags that can get set remotely, so we can do
stores and clearing of local-only flags without
atomic instructions?

> This way, unless I'm missing something, reduces the overhead to a 
> single store to a hot cacheline on return-to-userspace - which 
> instruction if we place it well might as well be close to zero cost. 
> No syscall entry cost. Slow-return cost only in the (rare) case of 
> someone using synchronize_rcu().

I think that should take care of the RCU aspect of
nohz_full.

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