Re: [PATCH 2/4] tile: support CONTEXT_TRACKING and thus NOHZ_FULL

[Chris Metcalf]

On 3/24/2015 5:15 PM, Frederic Weisbecker wrote:
> On Tue, Mar 24, 2015 at 03:21:33PM -0400, cmetcalf@xxxxxxxxxx wrote:
> > From: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> >
> > Add the TIF_NOHZ flag appropriately.
> >
> > Add call to user_exit() on entry to do_work_pending() and on entry
> > to syscalls via do_syscall_trace_enter(), and also the top of
> > do_syscall_trace_exit() just because it's done in x86.
> You only need to protect do_syscall_trace_exit() if there is a risk
> that something calls user_enter() before. x86 does it so because
> schedule_user() can be called before although I think we've changed
> schedule_user() to use exception_enter/exit. I should check if that
> user_exit() in do_syscall_trace_exit() is still necessary in x86.
>
> Anyway, calling user_exit() on context tracking kerne mode doesn't do
> any harm.

Yes, I see the exception_enter/exit in schedule_user().  I guess I will plan to
leave the user_exit() in for now, but if you conclude it's not necessary for x86,
I can remove it for tile as well at that point.

> > diff --git a/arch/tile/kernel/process.c b/arch/tile/kernel/process.c
> > index 48e5773dd0b7..b403c2e3e263 100644
> > --- a/arch/tile/kernel/process.c
> > +++ b/arch/tile/kernel/process.c
> > @@ -27,6 +27,7 @@
> >   #include <linux/kernel.h>
> >   #include <linux/tracehook.h>
> >   #include <linux/signal.h>
> > +#include <linux/context_tracking.h>
> >   #include <asm/stack.h>
> >   #include <asm/switch_to.h>
> >   #include <asm/homecache.h>
> > @@ -474,6 +475,8 @@ int do_work_pending(struct pt_regs *regs, u32 thread_info_flags)
> >   	if (!user_mode(regs))
> >   		return 0;
> >   
> > +	user_exit();
> > +
> >   	/* Enable interrupts; they are disabled again on return to caller. */
> >   	local_irq_enable();
> >   
> > @@ -496,11 +499,12 @@ int do_work_pending(struct pt_regs *regs, u32 thread_info_flags)
> >   		tracehook_notify_resume(regs);
> >   		return 1;
> >   	}
> > -	if (thread_info_flags & _TIF_SINGLESTEP) {
> > +	if (thread_info_flags & _TIF_SINGLESTEP)
> >   		single_step_once(regs);
> > -		return 0;
> > -	}
> > -	panic("work_pending: bad flags %#x\n", thread_info_flags);
> > +
> > +	user_enter();
> So, do work pending is called from syscall exit only? Or does it concern
> interrupts, exceptions as well?

It's called on every return to userspace if the TIF flags require it.

> Well if it's always followed by a return to userspace, it should be fine.

Let me know if you'd like me to put your Acked-by on the commit. Thanks!

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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