linux-kernel - Re: [PATCH, RFC] improved hacks to allow -rt to run kernbench on POWER

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Message-Id: <200711101218.47903.nickpiggin@yahoo.com.au>
Date:	Sat, 10 Nov 2007 12:18:47 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	benh@...nel.crashing.org
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	tony@...eyournoodle.com, paulus@...ba.org, dino@...ibm.com,
	tytso@...ibm.com, dvhltc@...ibm.com, niv@...ibm.com,
	antonb@...ibm.com, rostedt@...dmis.org
Subject: Re: [PATCH, RFC] improved hacks to allow -rt to run kernbench on POWER

On Saturday 10 November 2007 07:52, Benjamin Herrenschmidt wrote:
> > diff -urpNa -X dontdiff linux-2.6.23.1-rt4/arch/powerpc/kernel/process.c
> > linux-2.6.23.1-rt4-fix/arch/powerpc/kernel/process.c ---
> > linux-2.6.23.1-rt4/arch/powerpc/kernel/process.c	2007-10-12
> > 09:43:44.000000000 -0700 +++
> > linux-2.6.23.1-rt4-fix/arch/powerpc/kernel/process.c	2007-11-08
> > 20:33:59.000000000 -0800 @@ -245,6 +245,8 @@ struct task_struct
> > *__switch_to(struct t
> >  	struct thread_struct *new_thread, *old_thread;
> >  	unsigned long flags;
> >  	struct task_struct *last;
> > +	struct ppc64_tlb_batch *batch;
> > +	int hadbatch;
> >
> >  #ifdef CONFIG_SMP
> >  	/* avoid complexity of lazy save/restore of fpu
> > @@ -325,6 +327,16 @@ struct task_struct *__switch_to(struct t
> >  	}
> >  #endif
> >
> > +	batch = &get_cpu_var(ppc64_tlb_batch);
> > +	if (batch->active) {
> > +		hadbatch = 1;
> > +		if (batch->index) {
> > +			__flush_tlb_pending(batch);
> > +		}
> > +		batch->active = 0;
> > +	}
> > +	put_cpu_var(ppc64_tlb_batch);
> > +
> >  	local_irq_save(flags);
> >
> >  	account_system_vtime(current);
> > @@ -335,6 +347,12 @@ struct task_struct *__switch_to(struct t
> >
> >  	local_irq_restore(flags);
> >
> > +	if (hadbatch) {
> > +		batch = &get_cpu_var(ppc64_tlb_batch);
> > +		batch->active = 1;
> > +		put_cpu_var(ppc64_tlb_batch);
> > +	}
> > +
> >  	return last;
> >  }
>
> I doubt we can schedule within __switch_to() (can somebody confirm
> this ?), in which case, you can just use __get_cpu_var() and avoid
> the put, thus saving a handful of cycles in the code above.

Preempt is always turned off over switch_to() call.
-
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