RE: [PATCH 5/6] nohz: support PR_DATAPLANE_STRICT mode

[Gilad Ben Yossef]

> From: Andy Lutomirski [mailto:luto@xxxxxxxxxxxxxx]
> Sent: Saturday, May 09, 2015 10:29 AM
> To: Chris Metcalf
> Cc: Srivatsa S. Bhat; Paul E. McKenney; Frederic Weisbecker; Ingo Molnar;
> Rik van Riel; linux-doc@xxxxxxxxxxxxxxx; Andrew Morton; linux-
> kernel@xxxxxxxxxxxxxxx; Thomas Gleixner; Tejun Heo; Peter Zijlstra; Steven
> Rostedt; Christoph Lameter; Gilad Ben Yossef; Linux API
> Subject: Re: [PATCH 5/6] nohz: support PR_DATAPLANE_STRICT mode
> 
> On May 8, 2015 11:44 PM, "Chris Metcalf" <cmetcalf@xxxxxxxxxx> wrote:
> >
> > With QUIESCE mode, the task is in principle guaranteed not to be
> > interrupted by the kernel, but only if it behaves.  In particular,
> > if it enters the kernel via system call, page fault, or any of
> > a number of other synchronous traps, it may be unexpectedly
> > exposed to long latencies.  Add a simple flag that puts the process
> > into a state where any such kernel entry is fatal.
> >
> > To allow the state to be entered and exited, we add an internal
> > bit to current->dataplane_flags that is set when prctl() sets the
> > flags.  That way, when we are exiting the kernel after calling
> > prctl() to forbid future kernel exits, we don't get immediately
> > killed.
> 
> Is there any reason this can't already be addressed in userspace using
> /proc/interrupts or perf_events?  ISTM the real goal here is to detect
> when we screw up and fail to avoid an interrupt, and killing the task
> seems like overkill to me.
> 
> Also, can we please stop further torturing the exit paths?  
So, I don't know if it is a practical suggestion or not, but would it better/easier to mark a pending signal on kernel entry for this case?
The upsides I see is that the user gets her notification (killing the task or just logging the event in a signal handler) and hopefully since return to userspace with a pending signal is already handled we don't need new code in the exit path?

Gilad
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