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Date: Fri, 11 Sep 2015 10:46:36 +0200
From: Borislav Petkov <bp@alien8.de>
To: Ashok Raj <ashok.raj@intel.com>
Cc: linux-kernel@vger.kernel.org, linux-edac@vger.kernel.org,
        Tony Luck <tony.luck@intel.com>
Subject: Re: [Patch V1] x86, mce: CPU synchronization for broadcast MCE's is
 surprised by offline CPUs
Message-ID: <20150911084636.GB2042@pd.tnic>
References: <1441931198-3209-1-git-send-email-ashok.raj@intel.com>
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On Thu, Sep 10, 2015 at 08:26:38PM -0400, Ashok Raj wrote:
> +#define OFFLINE_CPU_LOG_LEN	16
> +
> +struct offline_cpu_mce {
> +	unsigned short head;
> +	unsigned short tail;
> +	struct mce mce_log[OFFLINE_CPU_LOG_LEN];
> +};
> +
> +static struct offline_cpu_mce offline_mce;
> +static unsigned int offline_mce_overflow = 0;
> +
> +/*
> + * Add mce's discovered in offline cpu which will be logged by the
> + * MCE rendezvous master. There is no lock required, since MCE's are
> + * processed one cpu at a time, sequenced by the rendezvous master CPU
> + * Safe to be called only from MCE handler.
> + */
> +static int offline_mce_add(struct mce *m)
> +{
> +	unsigned next;
> +
> +	next = (offline_mce.tail + 1) % OFFLINE_CPU_LOG_LEN;
> +	if (next == offline_mce.head) {
> +		offline_mce_overflow++;
> +		return -1;
> +	}
> +
> +	offline_mce.mce_log[offline_mce.tail] = *m;
> +	offline_mce.tail = next;
> +	return 0;
> +}
> +
> +static int offline_mce_get(struct mce *m)
> +{
> +	int ret = 0;
> +
> +	if (offline_mce.head == offline_mce.tail)
> +		goto out;
> +
> +	*m = offline_mce.mce_log[offline_mce.head];
> +	offline_mce.head = (offline_mce.head + 1) % OFFLINE_CPU_LOG_LEN;
> +
> +	ret = 1;
> +out:
> +	return ret;
> +}

One more buffer for MCEs? Why?

We did add the mce_gen_pool thing exactly for logging stuff in atomic
context. From looking at the code, we probably could get rid of that
"struct mce_log mcelog" thing too and use only the gen_pool for logging
MCEs.

We can then get rid of that MCE_LOG_LEN arbitrary 32 records and use
a nice 2-paged buffer which can be enlarged transparently later, if
needed.

Hmmm?

-- 
Regards/Gruss,
    Boris.

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