linux-kernel - Re: [PATCH] x86, UV: Fix RTC latency bug by reading replicated cachelines

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Message-ID: <20100120081020.GG20621@elte.hu>
Date:	Wed, 20 Jan 2010 09:10:20 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Dimitri Sivanich <sivanich@....com>
Cc:	linux-kernel@...r.kernel.org, Jack Steiner <steiner@....com>,
	"H. Peter Anvin" <hpa@...or.com>, tglx@...utronix.de
Subject: Re: [PATCH] x86, UV: Fix RTC latency bug by reading replicated
 cachelines


* Dimitri Sivanich <sivanich@....com> wrote:

> +++ linux/drivers/char/uv_mmtimer.c	2010-01-14 11:56:57.000000000 -0600
> @@ -89,12 +89,19 @@ static long uv_mmtimer_ioctl(struct file
>  	switch (cmd) {
>  	case MMTIMER_GETOFFSET:	/* offset of the counter */
>  		/*
> -		 * UV RTC register is on its own page
> +		 * Starting with HUB rev 2.0, the UV RTC register is
> +		 * replicated across all cachelines of it's own page.
> +		 * This allows faster simultaneous reads from a given socket.
> +		 *
> +		 * The offset returned is in 64 bit units.
>  		 */
> -		if (PAGE_SIZE <= (1 << 16))
> -			ret = ((UV_LOCAL_MMR_BASE | UVH_RTC) & (PAGE_SIZE-1))
> -				/ 8;
> -		else
> +		if (PAGE_SIZE <= (1 << 16)) {
> +			if (uv_get_min_hub_revision_id() == 1)
> +				ret = 0;
> +			else
> +				ret = ((uv_blade_processor_id() *
> +					L1_CACHE_BYTES) % PAGE_SIZE) / 8;
> +		} else

That 64K PAGE_SIZE check in the ioctl looks rather weird. What is the purpose 
of it?

	Ingo
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