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

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Message-ID: <alpine.DEB.2.21.1911120816570.1833@nanos.tec.linutronix.de>
Date:   Tue, 12 Nov 2019 08:17:47 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
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

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/ :)
 
> s/tasks
>  /task's

Thanks,

	tglx