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Date: Wed, 2 May 2007 17:00:01 -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 (try #2)
Newer version coming in a while. Testing.
On 4/30/07, 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
> situations where accurate accounting of MCEs is important. 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 or exit from idle, 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 per poll, 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.
>
> I also considered doing the MCE poll directly from the idle notifier, but
> decided that was overkill.
>
> 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.
>
> Caveats:
> I have seen a soft lockup with a call trace always similar to:
> 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
>
> I regressed this to the vanilla kernel, and it still happens. It only
> crops up in the face of multiple uncorrectable errors.
>
> Patch:
> This patch is against 2.6.21-rc7.
>
> Signed-off-by: Tim Hockin <thockin@...gle.com>
>
> ---
>
> This is the second version version of this patch. The TIF_* approach was
> suggested by Mike Waychison and Andi did not yell at me when I suggested
> it. Hooking the idle notifier was born of an Andrew Morton suggestion
> and, no surprise, seems to work well.
>
>
> diff -pruN linux-2.6.20+th/arch/x86_64/kernel/entry.S linux-2.6.20+th2v3/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+th2v3/arch/x86_64/kernel/entry.S 2007-04-30 10:57:43.000000000 -0700
> @@ -282,7 +282,7 @@ sysret_careful:
> sysret_signal:
> TRACE_IRQS_ON
> sti
> - testl $(_TIF_SIGPENDING|_TIF_NOTIFY_RESUME|_TIF_SINGLESTEP),%edx
> + testl $(_TIF_SIGPENDING|_TIF_NOTIFY_RESUME|_TIF_SINGLESTEP|_TIF_MCE_NOTIFY),%edx
> jz 1f
>
> /* Really a signal */
> @@ -375,7 +375,7 @@ int_very_careful:
> jmp int_restore_rest
>
> int_signal:
> - testl $(_TIF_NOTIFY_RESUME|_TIF_SIGPENDING|_TIF_SINGLESTEP),%edx
> + testl $(_TIF_NOTIFY_RESUME|_TIF_SIGPENDING|_TIF_SINGLESTEP|_TIF_MCE_NOTIFY),%edx
> jz 1f
> movq %rsp,%rdi # &ptregs -> arg1
> xorl %esi,%esi # oldset -> arg2
> @@ -597,9 +597,9 @@ retint_careful:
> cli
> TRACE_IRQS_OFF
> jmp retint_check
> -
> +
> retint_signal:
> - testl $(_TIF_SIGPENDING|_TIF_NOTIFY_RESUME|_TIF_SINGLESTEP),%edx
> + testl $(_TIF_SIGPENDING|_TIF_NOTIFY_RESUME|_TIF_SINGLESTEP|_TIF_MCE_NOTIFY),%edx
> jz retint_swapgs
> TRACE_IRQS_ON
> sti
> diff -pruN linux-2.6.20+th/arch/x86_64/kernel/mce.c linux-2.6.20+th2v3/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+th2v3/arch/x86_64/kernel/mce.c 2007-04-30 22:19:25.000000000 -0700
> @@ -20,12 +20,15 @@
> #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>
> #include <asm/kdebug.h>
> #include <asm/uaccess.h>
> #include <asm/smp.h>
> +#include <asm/idle.h>
>
> #define MISC_MCELOG_MINOR 227
> #define NR_BANKS 6
> @@ -39,8 +42,7 @@ 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 +50,8 @@ 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 +98,7 @@ 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 +170,21 @@ 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 and idle, thanks to TIF_MCE_NOTIFY.
> + */
> +int 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);
> - call_usermodehelper(trigger, trigger_argv, NULL, -1);
> + clear_thread_flag(TIF_MCE_NOTIFY);
> + if (test_and_clear_bit(0, ¬ify_user)) {
> + wake_up_interruptible(&mce_wait);
> + if (trigger[0])
> + call_usermodehelper(trigger, trigger_argv, NULL, -1);
> + return 1;
> }
> + return 0;
> }
>
> /*
> @@ -251,12 +260,8 @@ 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 +293,9 @@ 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);
> @@ -343,21 +351,13 @@ 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.
> - */
> - if (notify_user && console_logged) {
> + /* alert userspace if needed */
> + if (mce_notify_user()) {
> static unsigned long last_print;
> unsigned long now = jiffies;
>
> /* 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");
> @@ -369,12 +369,27 @@ static void mcheck_timer(struct work_str
> schedule_delayed_work(&mcheck_work, next_interval);
> }
>
> +/* see if the idle task needs to notify userspace */
> +static int
> +mce_idle_callback(struct notifier_block *nfb, unsigned long action, void *junk)
> +{
> + /* IDLE_END should be safe - interrupts are back on */
> + if (action == IDLE_END && test_thread_flag(TIF_MCE_NOTIFY))
> + mce_notify_user();
> +
> + return NOTIFY_OK;
> +}
> +
> +static struct notifier_block mce_idle_notifier = {
> + .notifier_call = mce_idle_callback,
> +};
>
> static __init int periodic_mcheck_init(void)
> {
> next_interval = check_interval * HZ;
> if (next_interval)
> schedule_delayed_work(&mcheck_work, next_interval);
> + idle_notifier_register(&mce_idle_notifier);
> return 0;
> }
> __initcall(periodic_mcheck_init);
> @@ -530,6 +545,14 @@ 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 +577,7 @@ 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/arch/x86_64/kernel/signal.c linux-2.6.20+th2v3/arch/x86_64/kernel/signal.c
> --- linux-2.6.20+th/arch/x86_64/kernel/signal.c 2007-02-04 10:44:54.000000000 -0800
> +++ linux-2.6.20+th2v3/arch/x86_64/kernel/signal.c 2007-04-30 10:49:11.000000000 -0700
> @@ -27,6 +27,7 @@
> #include <asm/i387.h>
> #include <asm/proto.h>
> #include <asm/ia32_unistd.h>
> +#include <asm/mce.h>
>
> /* #define DEBUG_SIG 1 */
>
> @@ -473,6 +474,10 @@ do_notify_resume(struct pt_regs *regs, v
> clear_thread_flag(TIF_SINGLESTEP);
> }
>
> + /* notify userspace of pending MCEs */
> + if (thread_info_flags & _TIF_MCE_NOTIFY)
> + mce_notify_user();
> +
> /* deal with pending signal delivery */
> if (thread_info_flags & (_TIF_SIGPENDING|_TIF_RESTORE_SIGMASK))
> do_signal(regs);
> diff -pruN linux-2.6.20+th/include/asm-x86_64/mce.h linux-2.6.20+th2v3/include/asm-x86_64/mce.h
> --- linux-2.6.20+th/include/asm-x86_64/mce.h 2007-04-24 22:46:22.000000000 -0700
> +++ linux-2.6.20+th2v3/include/asm-x86_64/mce.h 2007-04-30 21:53:09.000000000 -0700
> @@ -105,6 +105,8 @@ extern atomic_t mce_entry;
>
> extern void do_machine_check(struct pt_regs *, long);
>
> +extern int mce_notify_user(void);
> +
> #endif
>
> #endif
> diff -pruN linux-2.6.20+th/include/asm-x86_64/thread_info.h linux-2.6.20+th2v3/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+th2v3/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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