Date:   Thu, 19 Oct 2023 17:12:11 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tony Luck <tony.luck@...el.com>
Cc:     Yazen Ghannam <yazen.ghannam@....com>,
        Smita.KoralahalliChannabasappa@....com,
        dave.hansen@...ux.intel.com, x86@...nel.org,
        linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH v9 2/3] x86/mce: Add per-bank CMCI storm mitigation

On Wed, Oct 04, 2023 at 11:36:22AM -0700, Tony Luck wrote:
> +/*
> + * history:	bitmask tracking whether errors were seen or not seen in
> + *		the most recent polls of a bank.

		each bit in that bitmask represents an error seen.

> + * timestamp:	last time (in jiffies) that the bank was polled
> + * storm:	Is this bank in storm mode?
> + */
> +struct storm_bank {
> +	u64 history;
> +	u64 timestamp;
> +	bool storm;

I guess "in_storm_mode" is even more descriptive:

	storm->banks[bank].in_storm_mode = false;

etc.

> +};
> +
> +/* How many errors within the history buffer mark the start of a storm. */
> +#define STORM_BEGIN_THRESHOLD	5
> +
> +/*
> + * How many polls of machine check bank without an error before declaring
> + * the storm is over. Since it is tracked by the bitmaks in the history
> + * field of struct storm_bank the mask is 30 bits [0 ... 29].
> + */
> +#define STORM_END_POLL_THRESHOLD	29
>  
>  #ifdef CONFIG_ACPI_APEI
>  int apei_write_mce(struct mce *m);
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index f6e87443b37a..7c931f0c9251 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -680,6 +680,8 @@ bool machine_check_poll(enum mcp_flags flags, mce_banks_t *b)
>  		barrier();
>  		m.status = mce_rdmsrl(mca_msr_reg(i, MCA_STATUS));
>  
> +		mce_track_storm(&m);

Lemme see if I understand the idea here:

the default polling interval is 5 mins. So the storm tracking is called
every 5 mins once to see how are banks "doing", so to speak and to get
them in or out of storm mode. So far so good...

> +
>  		/* If this entry is not valid, ignore it */
>  		if (!(m.status & MCI_STATUS_VAL))
>  			continue;

Btw, you're tracking storm even if the error is not valid - conditional
above here. Why?

> @@ -1652,22 +1654,29 @@ static void mce_timer_fn(struct timer_list *t)
>  	else
>  		iv = min(iv * 2, round_jiffies_relative(check_interval * HZ));
>  
> -	__this_cpu_write(mce_next_interval, iv);
> -	__start_timer(t, iv);
> +	if (mce_is_storm()) {
> +		__start_timer(t, HZ);
> +	} else {
> +		__this_cpu_write(mce_next_interval, iv);
> +		__start_timer(t, iv);
> +	}

... this is where it becomes, hm, interesting: the check interval will
be halved if an error has been seen during this round but then if we're
in storm mode, that check interval doesn't matter - you'll run the timer
each second.

Then you need to restructure this to check the storm condition and not
do anything to iv if storm.

Or, am I missing something?

> diff --git a/arch/x86/kernel/cpu/mce/threshold.c b/arch/x86/kernel/cpu/mce/threshold.c
> index ef4e7bb5fd88..ecdf13f1bb7d 100644
> --- a/arch/x86/kernel/cpu/mce/threshold.c
> +++ b/arch/x86/kernel/cpu/mce/threshold.c
> @@ -29,3 +29,115 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_threshold)
>  	trace_threshold_apic_exit(THRESHOLD_APIC_VECTOR);
>  	apic_eoi();
>  }
> +
> +/*
> + * banks:		per-cpu, per-bank details
> + * stormy_bank_count:	count of MC banks in storm state
> + * poll_mode:		CPU is in poll mode
> + */
> +struct mca_storm_desc {
> +	struct storm_bank	banks[MAX_NR_BANKS];
> +	u8			stormy_bank_count;
> +	bool			poll_mode;
> +};

Yeah, put the struct definition into internal.h pls.

> +static DEFINE_PER_CPU(struct mca_storm_desc, storm_desc);
> +
> +void mce_inherit_storm(unsigned int bank)
> +{
> +	struct mca_storm_desc *storm = this_cpu_ptr(&storm_desc);
> +
> +	storm->banks[bank].history = ~0ull;

So upon inheriting a bank, you set its history that it has seen errors
each time?

That's weird.

> +	storm->banks[bank].timestamp = jiffies;
> +}
> +
> +bool mce_is_storm(void)

That's a weird name. mce_get_storm_mode() perhaps?

> +{
> +	return __this_cpu_read(storm_desc.poll_mode);
> +}
> +
> +void mce_set_storm(bool storm)

mce_set_storm_mode()

> +{
> +	__this_cpu_write(storm_desc.poll_mode, storm);
> +}
> +
> +static void mce_handle_storm(unsigned int bank, bool on)
> +{
> +	switch (boot_cpu_data.x86_vendor) {
> +	}
> +}

...

> +void mce_track_storm(struct mce *mce)
> +{
> +	struct mca_storm_desc *storm = this_cpu_ptr(&storm_desc);
> +	unsigned long now = jiffies, delta;
> +	unsigned int shift = 1;
> +	u64 history = 0;
> +
> +	/*
> +	 * When a bank is in storm mode it is polled once per second and
> +	 * the history mask will record about the last minute of poll results.
> +	 * If it is not in storm mode, then the bank is only checked when
> +	 * there is a CMCI interrupt. Check how long it has been since
> +	 * this bank was last checked, and adjust the amount of "shift"
> +	 * to apply to history.
> +	 */
> +	if (!storm->banks[mce->bank].storm) {

Yeah, if this were

	if (!storm->banks[mce->bank].in_storm_mode)

it would've been perfectly clear what the condition tests.

> +		delta = now - storm->banks[mce->bank].timestamp;
> +		shift = (delta + HZ) / HZ;
> +	}
> +
> +	/* If it has been a long time since the last poll, clear history. */
> +	if (shift < 64)

Use a properly named define instead of a naked number.

...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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