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SubjectRe: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical

* Thomas Gleixner <tglx@linutronix.de> wrote:

> On Tue, 12 Nov 2019, Ingo Molnar wrote:
> > * Thomas Gleixner <tglx@linutronix.de> wrote:
> > > +void io_bitmap_share(struct task_struct *tsk)
> > > + {
> > > + /*
> > > + * Take a refcount on current's bitmap. It can be used by
> > > + * both tasks as long as none of them changes the bitmap.
> > > + */
> > > + refcount_inc(&current->thread.io_bitmap->refcnt);
> > > + tsk->thread.io_bitmap = current->thread.io_bitmap;
> > > + set_tsk_thread_flag(tsk, TIF_IO_BITMAP);
> > > +}
> >
> > Ok, this is really neat. I suspect there might be some pathological cases
> > on ancient NUMA systems with a really high NUMA factor and bad caching
> > where this new sharing might regress performance, but I doubt this
> > matters, as both the hardware and this software functionality is legacy.
>
> Definitely.
>
> > > + /*
> > > + * If the bitmap is not shared, then nothing can take a refcount as
> > > + * current can obviously not fork at the same time. If it's shared
> > > + * duplicate it and drop the refcount on the original one.
> > > + */
> > > + if (refcount_read(&iobm->refcnt) > 1) {
> > > + iobm = kmemdup(iobm, sizeof(*iobm), GFP_KERNEL);
> > > + if (!iobm)
> > > + return -ENOMEM;
> > > + io_bitmap_exit();
> > > }
> > >
> > > + /* Set the tasks io_bitmap pointer (might be the same) */
> >
> > speling nit:
>
> s/speling/spelling/ :)

Was a lame attempt at a self-depreciating joke ;-)

Thanks,

Ingo

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