linux-kernel - Re: [PATCH 03/18] xen/pvcalls: initialize the module and register the xenbus backend

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Message-ID: <alpine.DEB.2.10.1705161249000.4103@sstabellini-ThinkPad-X260>
Date:   Tue, 16 May 2017 12:58:02 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Juergen Gross <jgross@...e.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        boris.ostrovsky@...cle.com,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH 03/18] xen/pvcalls: initialize the module and register
 the xenbus backend

On Tue, 16 May 2017, Juergen Gross wrote:
> On 15/05/17 22:35, Stefano Stabellini wrote:
> > The pvcalls backend has one ioworker per cpu: the ioworkers are
> > implemented as a cpu bound workqueue, and will deal with the actual
> > socket and data ring reads/writes.
> > 
> > ioworkers are global: we only have one set for all the frontends. They
> > process requests on their wqs list in order, once they are done with a
> > request, they'll remove it from the list. A spinlock is used for
> > protecting the list. Each ioworker is bound to a different cpu to
> > maximize throughput.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> > CC: boris.ostrovsky@...cle.com
> > CC: jgross@...e.com
> > ---
> >  drivers/xen/pvcalls-back.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 64 insertions(+)
> > 
> > diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
> > index 2dbf7d8..46a889a 100644
> > --- a/drivers/xen/pvcalls-back.c
> > +++ b/drivers/xen/pvcalls-back.c
> > @@ -25,6 +25,26 @@
> >  #include <xen/xenbus.h>
> >  #include <xen/interface/io/pvcalls.h>
> >  
> > +struct pvcalls_ioworker {
> > +	struct work_struct register_work;
> > +	atomic_t io;
> > +	struct list_head wqs;
> > +	spinlock_t lock;
> > +	int num;
> > +};
> > +
> > +struct pvcalls_back_global {
> > +	struct pvcalls_ioworker *ioworkers;
> > +	int nr_ioworkers;
> > +	struct workqueue_struct *wq;
> > +	struct list_head privs;
> > +	struct rw_semaphore privs_lock;
> > +} pvcalls_back_global;
> > +
> > +static void pvcalls_back_ioworker(struct work_struct *work)
> > +{
> > +}
> > +
> >  static int pvcalls_back_probe(struct xenbus_device *dev,
> >  			      const struct xenbus_device_id *id)
> >  {
> > @@ -59,3 +79,47 @@ static int pvcalls_back_uevent(struct xenbus_device *xdev,
> >  	.uevent = pvcalls_back_uevent,
> >  	.otherend_changed = pvcalls_back_changed,
> >  };
> > +
> > +static int __init pvcalls_back_init(void)
> > +{
> > +	int ret, i, cpu;
> > +
> > +	if (!xen_domain())
> > +		return -ENODEV;
> > +
> > +	ret = xenbus_register_backend(&pvcalls_back_driver);
> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	init_rwsem(&pvcalls_back_global.privs_lock);
> > +	INIT_LIST_HEAD(&pvcalls_back_global.privs);
> > +	pvcalls_back_global.wq = alloc_workqueue("pvcalls_io", 0, 0);
> > +	if (!pvcalls_back_global.wq)
> > +		goto error;
> > +	pvcalls_back_global.nr_ioworkers = num_online_cpus();
> 
> Really? Recently I cam across a system with 640 dom0 cpus. I don't think
> we want 640 workers initialized when loading the backend module. I'd
> prefer one or a few workers per connected frontend.

I think we want to keep the ioworker allocation to be based on the
number of vcpus: we do not want more ioworkers than vcpus because it is
a waste of resources and leads to worse performance.  Also, given that
they do memcpy's, I also think it is a good idea to bind them to vcpus
(and pin vcpus to pcpus) to get best performance.

However, you have a point there: we need to handle systems with an
extremely large number of Dom0 vcpus. I suggest we introduce an
upper limit for the number of ioworkers. Something like:

#define MAX_IOWORKERS 64
nr_ioworkers = min(MAX_IOWORKERS, num_online_cpus())

MAX_IOWORKERS could be configurable via a command line option.


> > +	pvcalls_back_global.ioworkers = kzalloc(
> > +		sizeof(*pvcalls_back_global.ioworkers) *
> > +		pvcalls_back_global.nr_ioworkers, GFP_KERNEL);
> 
> kcalloc()?

I'll make the change


> > +	if (!pvcalls_back_global.ioworkers)
> > +		goto error;
> > +	i = 0;
> > +	for_each_online_cpu(cpu) {
> > +		pvcalls_back_global.ioworkers[i].num = i;
> > +		atomic_set(&pvcalls_back_global.ioworkers[i].io, 1);
> > +		spin_lock_init(&pvcalls_back_global.ioworkers[i].lock);
> > +		INIT_LIST_HEAD(&pvcalls_back_global.ioworkers[i].wqs);
> > +		INIT_WORK(&pvcalls_back_global.ioworkers[i].register_work,
> > +			pvcalls_back_ioworker);
> > +		i++;
> > +	}
> > +	return 0;
> > +
> > +error:
> > +	if (pvcalls_back_global.wq)
> > +		destroy_workqueue(pvcalls_back_global.wq);
> > +	xenbus_unregister_driver(&pvcalls_back_driver);
> > +	kfree(pvcalls_back_global.ioworkers);
> > +	memset(&pvcalls_back_global, 0, sizeof(pvcalls_back_global));
> > +	return -ENOMEM;
> > +}
> > +module_init(pvcalls_back_init);
> > 
> 
> Juergen
>