linux-kernel - Re: [PATCH 2/6] sched: add function to execute a function synchronously on a physical cpu

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Date:	Fri, 11 Mar 2016 13:57:06 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juergen Gross <jgross@...e.com>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
	david.vrabel@...rix.com, mingo@...hat.com,
	Douglas_Warzecha@...l.com, pali.rohar@...il.com, jdelvare@...e.com,
	linux@...ck-us.net, tglx@...utronix.de, hpa@...or.com,
	x86@...nel.org
Subject: Re: [PATCH 2/6] sched: add function to execute a function
 synchronously on a physical cpu

On Fri, Mar 11, 2016 at 01:48:12PM +0100, Juergen Gross wrote:
> On 11/03/16 13:42, Peter Zijlstra wrote:
> > how about something like:
> > 
> > struct xen_callback_struct {
> > 	struct work_struct	work;
> > 	struct completion	done;
	int			(*func)(void*);
> > 	void *			data;
> > 	int			ret;
> > };
> > 
> > static void xen_callback_f(struct work_struct *work)
> > {
> > 	struct xen_callback_struct *xcs = container_of(work, struct xen_callback_struct, work);
> > 
> > 	xcs->ret = xcs->func(xcs->data);
> > 
> > 	complete(&xcs->done);
> > }
> > 
> > xen_call_on_cpu_sync(int cpu, int (*func)(void *), void *data)
> > {
> > 	struct xen_callback_state xcs = {
> > 		.work = __WORK_INITIALIZER(xcs.work, xen_callback_f);
> > 		.done = COMPLETION_INITIALIZER_ONSTACK(xcs.done),
		.func = func,
> > 		.data = data,
> > 	};
> > 
> > 	queue_work_on(&work, cpu);
> > 	wait_for_completion(&xcs.done);
> > 
> > 	return xcs.ret;
> > }
> > 
> > No mucking about with the scheduler state, no new exported functions
> > etc..
> > 
> 
> Hey, I like it. Can't be limited to Xen as on bare metal the function
> needs to be called on cpu 0, too. But avoiding the scheduler fiddling
> is much better! As this seems to be required for Dell hardware only,
> I could add it to some Dell base driver in case you don't want to add
> it to core code.

Urgh yeah, saw that in your other mail. It looks like I should go look
at set_cpus_allowed_ptr() abuse :/

Not sure where this would fit best, maybe somewhere near workqueue.c or
smp.c.