linux-kernel - Re: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical

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Date:   Tue, 12 Nov 2019 08:52:35 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V2 11/16] x86/ioperm: Share I/O bitmap if identical


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

> On Tue, 12 Nov 2019, Ingo Molnar wrote:
> > * Thomas Gleixner <tglx@...utronix.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