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Date: Sun, 29 Apr 2007 20:48:20 -0700
From: "Tim Hockin" <thockin@...gle.com>
To: ak@...e.de, vojtech@...e.cz
Cc: linux-kernel@...r.kernel.org, akpm@...gle.com
Subject: Re: [PATCH] x86_64: support poll() on /dev/mcelog
Crap - I have just got soft lockup detected. Back to debug.
Call Trace:
<IRQ> [<ffffffff80227e80>] wake_up_process+0x10/0x20
[<ffffffff8025361a>] softlockup_tick+0xea/0x110
[<ffffffff80235d53>] run_local_timers+0x13/0x20
[<ffffffff80235e67>] update_process_times+0x57/0x90
[<ffffffff802116b0>] mcheck_check_cpu+0x0/0x40
[<ffffffff80214bd4>] smp_local_timer_interrupt+0x34/0x60
[<ffffffff8021538e>] smp_apic_timer_interrupt+0x4e/0x70
[<ffffffff8020a436>] apic_timer_interrupt+0x66/0x70
On 4/29/07, Tim Hockin <thockin@...gle.com> wrote:
> From: Tim Hockin <thockin@...gle.com>
>
> Background:
> /dev/mcelog is typically polled manually. This is less than optimal for
> some situations. Calling poll() on /dev/mcelog does not work.
>
> Description:
> This patch adds support for poll() to /dev/mcelog. This results in
> immediate wakeup of user apps whenever the poller finds MCEs. Because
> the exception handler can not take any locks, it can not call the wakeup
> itself. Instead, it uses a thread_info flag (TIF_MCE_NOTIFY) which is
> caught at the next return from interrupt, calling the mce_user_notify()
> routine.
>
> This patch also does some small cleanup for essentially unused variables,
> and moves the user notification into the body of the poller, so it is
> only called once, rather than once per CPU.
>
> Result:
> Applications can now poll() on /dev/mcelog. When an error is logged
> (whether through the poller or through an exception) the applications are
> woken up promptly. This should not affect any previous behaviors. If no
> MCEs are being logged, there is no overhead.
>
> Alternatives:
> I considered simply supporting poll() through the poller and not using
> TIF_MCE_NOTIFY at all. However, the time between an uncorrectable error
> happening and the user application being notified is *the*most* critical
> window for us. Many uncorrectable errors can be logged to the network if
> given a chance.
>
> Testing:
> I used an error-injecting DIMM to create lots of correctable DRAM errors
> and verified that my user app is woken up in sync with the polling interval.
> I also used the northbridge to inject uncorrectable ECC errors, and
> verified (printk() to the rescue) that the notify routine is called and the
> user app does wake up.
>
> Patch:
> This patch is against 2.6.21-rc7.
>
> Signed-Off-By: Tim Hockin <thockin@...gle.com>
>
> ---
>
> This is the first public version version of this patch. The TIF_*
> approach was suggested by Mike Waychison and Andi did not yell at me when
> I suggested it. :)
>
>
> diff -pruN linux-2.6.20+th/arch/x86_64/kernel/entry.S linux-2.6.20+th2v2/arch/x86_64/kernel/entry.S
> --- linux-2.6.20+th/arch/x86_64/kernel/entry.S 2007-04-24 22:46:19.000000000 -0700
> +++ linux-2.6.20+th2v2/arch/x86_64/kernel/entry.S 2007-04-29 18:10:40.000000000 -0700
> @@ -585,7 +585,7 @@ bad_iret:
> retint_careful:
> CFI_RESTORE_STATE
> bt $TIF_NEED_RESCHED,%edx
> - jnc retint_signal
> + jnc retint_mce
> TRACE_IRQS_ON
> sti
> pushq %rdi
> @@ -597,7 +597,23 @@ retint_careful:
> cli
> TRACE_IRQS_OFF
> jmp retint_check
> -
> +
> + /* handle a waiting machine check */
> +retint_mce:
> + bt $TIF_MCE_NOTIFY,%edx
> + jnc retint_signal
> + TRACE_IRQS_ON
> + sti
> + pushq %rdi
> + CFI_ADJUST_CFA_OFFSET 8
> + call mce_notify_user
> + popq %rdi
> + CFI_ADJUST_CFA_OFFSET -8
> + GET_THREAD_INFO(%rcx)
> + cli
> + TRACE_IRQS_OFF
> + jmp retint_check
> +
> retint_signal:
> testl $(_TIF_SIGPENDING|_TIF_NOTIFY_RESUME|_TIF_SINGLESTEP),%edx
> jz retint_swapgs
> diff -pruN linux-2.6.20+th/arch/x86_64/kernel/mce.c linux-2.6.20+th2v2/arch/x86_64/kernel/mce.c
> --- linux-2.6.20+th/arch/x86_64/kernel/mce.c 2007-04-27 14:19:08.000000000 -0700
> +++ linux-2.6.20+th2v2/arch/x86_64/kernel/mce.c 2007-04-29 18:27:22.000000000 -0700
> @@ -20,6 +20,8 @@
> #include <linux/percpu.h>
> #include <linux/ctype.h>
> #include <linux/kmod.h>
> +#include <linux/poll.h>
> +#include <linux/thread_info.h>
> #include <asm/processor.h>
> #include <asm/msr.h>
> #include <asm/mce.h>
> @@ -39,8 +41,7 @@ static int mce_dont_init;
> static int tolerant = 1;
> static int banks;
> static unsigned long bank[NR_BANKS] = { [0 ... NR_BANKS-1] = ~0UL };
> -static unsigned long console_logged;
> -static int notify_user;
> +static unsigned long notify_user;
> static int rip_msr;
> static int mce_bootlog = 1;
> static atomic_t mce_events;
> @@ -48,6 +49,8 @@ static atomic_t mce_events;
> static char trigger[128];
> static char *trigger_argv[2] = { trigger, NULL };
>
> +static DECLARE_WAIT_QUEUE_HEAD(mce_wait);
> +
> /*
> * Lockless MCE logging infrastructure.
> * This avoids deadlocks on printk locks without having to break locks. Also
> @@ -94,8 +97,7 @@ void mce_log(struct mce *mce)
> mcelog.entry[entry].finished = 1;
> wmb();
>
> - if (!test_and_set_bit(0, &console_logged))
> - notify_user = 1;
> + set_bit(0, ¬ify_user);
> }
>
> static void print_mce(struct mce *m)
> @@ -167,15 +169,19 @@ static inline void mce_get_rip(struct mc
> }
> }
>
> -static void do_mce_trigger(void)
> +/*
> + * This is only called in normal interrupt context. This is where we do
> + * anything we need to alert userspace. This is called directly from the
> + * poller and also from entry.S thanks to TIF_MCE_NOTIFY.
> + */
> +void mce_notify_user(void)
> {
> - static atomic_t mce_logged;
> - int events = atomic_read(&mce_events);
> - if (events != atomic_read(&mce_logged) && trigger[0]) {
> - /* Small race window, but should be harmless. */
> - atomic_set(&mce_logged, events);
> + clear_thread_flag(TIF_MCE_NOTIFY);
> + clear_bit(0, ¬ify_user);
> +
> + wake_up_interruptible(&mce_wait);
> + if (trigger[0])
> call_usermodehelper(trigger, trigger_argv, NULL, -1);
> - }
> }
>
> /*
> @@ -251,12 +257,8 @@ void do_machine_check(struct pt_regs * r
> }
>
> /* Never do anything final in the polling timer */
> - if (!regs) {
> - /* Normal interrupt context here. Call trigger for any new
> - events. */
> - do_mce_trigger();
> + if (!regs)
> goto out;
> - }
>
> /* If we didn't find an uncorrectable error, pick
> the last one (shouldn't happen, just being safe). */
> @@ -288,6 +290,9 @@ void do_machine_check(struct pt_regs * r
> do_exit(SIGBUS);
> }
>
> + /* notify userspace ASAP */
> + set_thread_flag(TIF_MCE_NOTIFY);
> +
> out:
> /* Last thing done in the machine check exception to clear state. */
> wrmsrl(MSR_IA32_MCG_STATUS, 0);
> @@ -344,20 +349,18 @@ static void mcheck_timer(struct work_str
> on_each_cpu(mcheck_check_cpu, NULL, 1, 1);
>
> /*
> - * It's ok to read stale data here for notify_user and
> - * console_logged as we'll simply get the updated versions
> - * on the next mcheck_timer execution and atomic operations
> - * on console_logged act as synchronization for notify_user
> - * writes.
> + * It's ok to read stale data here for notify_user as we'll simply
> + * get the updated version on the next mcheck_timer execution.
> */
> - if (notify_user && console_logged) {
> + if (notify_user) {
> static unsigned long last_print;
> unsigned long now = jiffies;
>
> + /* alert userspace */
> + mce_notify_user();
> +
> /* if we logged an MCE, reduce the polling interval */
> next_interval = max(next_interval/2, HZ/100);
> - notify_user = 0;
> - clear_bit(0, &console_logged);
> if (time_after_eq(now, last_print + (check_interval*HZ))) {
> last_print = now;
> printk(KERN_INFO "Machine check events logged\n");
> @@ -530,6 +533,14 @@ static ssize_t mce_read(struct file *fil
> return err ? -EFAULT : buf - ubuf;
> }
>
> +static unsigned int mce_poll(struct file *file, poll_table *wait)
> +{
> + poll_wait(file, &mce_wait, wait);
> + if (rcu_dereference(mcelog.next))
> + return POLLIN | POLLRDNORM;
> + return 0;
> +}
> +
> static int mce_ioctl(struct inode *i, struct file *f,unsigned int cmd, unsigned long arg)
> {
> int __user *p = (int __user *)arg;
> @@ -554,6 +565,7 @@ static int mce_ioctl(struct inode *i, st
>
> static const struct file_operations mce_chrdev_ops = {
> .read = mce_read,
> + .poll = mce_poll,
> .ioctl = mce_ioctl,
> };
>
> diff -pruN linux-2.6.20+th/include/asm-x86_64/thread_info.h linux-2.6.20+th2v2/include/asm-x86_64/thread_info.h
> --- linux-2.6.20+th/include/asm-x86_64/thread_info.h 2007-02-04 10:44:54.000000000 -0800
> +++ linux-2.6.20+th2v2/include/asm-x86_64/thread_info.h 2007-04-27 23:13:59.000000000 -0700
> @@ -115,6 +115,7 @@ static inline struct thread_info *stack_
> #define TIF_SYSCALL_AUDIT 7 /* syscall auditing active */
> #define TIF_SECCOMP 8 /* secure computing */
> #define TIF_RESTORE_SIGMASK 9 /* restore signal mask in do_signal */
> +#define TIF_MCE_NOTIFY 10 /* notify userspace of an MCE */
> /* 16 free */
> #define TIF_IA32 17 /* 32bit process */
> #define TIF_FORK 18 /* ret_from_fork */
> @@ -133,6 +134,7 @@ static inline struct thread_info *stack_
> #define _TIF_SYSCALL_AUDIT (1<<TIF_SYSCALL_AUDIT)
> #define _TIF_SECCOMP (1<<TIF_SECCOMP)
> #define _TIF_RESTORE_SIGMASK (1<<TIF_RESTORE_SIGMASK)
> +#define _TIF_MCE_NOTIFY (1<<TIF_MCE_NOTIFY)
> #define _TIF_IA32 (1<<TIF_IA32)
> #define _TIF_FORK (1<<TIF_FORK)
> #define _TIF_ABI_PENDING (1<<TIF_ABI_PENDING)
>
-
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