linux-kernel - Re: [patch V2 08/16] x86/ioperm: Add bitmap sequence number

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Date:   Tue, 12 Nov 2019 10:22:46 +0100
From:   Peter Zijlstra <peterz@...radead.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>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch V2 08/16] x86/ioperm: Add bitmap sequence number

On Mon, Nov 11, 2019 at 11:03:22PM +0100, Thomas Gleixner wrote:
> Add a globally unique sequence number which is incremented when ioperm() is
> changing the I/O bitmap of a task. Store the new sequence number in the
> io_bitmap structure and compare it along with the actual struct pointer
> with the one which was last loaded on a CPU. Only update the bitmap if
> either of the two changes. That should further reduce the overhead of I/O
> bitmap scheduling when there are only a few I/O bitmap users on the system.

> +	/* Update the sequence number to force an update in switch_to() */
> +	iobm->sequence = atomic64_add_return(1, &io_bitmap_sequence);

> +		if (tss->last_bitmap != iobm ||
> +		    tss->last_sequence != iobm->sequence)
> +			switch_to_update_io_bitmap(tss, iobm);

Initially I wondered why we need a globally unique sequence number if we
already check the struct iobitmap pointer. That ought to make the
sequence number per-object.

However, that breaks for memory re-use. So yes, we need that thing to be
global.