Messages in this thread | | | From | John Ogness <> | Subject | Re: [PATCH] lib/nmi_backtrace: Serialize even messages about idle CPUs | Date | Tue, 27 Jul 2021 17:53:04 +0206 |
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On 2021-07-27, Petr Mladek <pmladek@suse.com> wrote: > The commit 55d6af1d66885059ffc2a ("lib/nmi_backtrace: explicitly serialize > banner and regs") serialized backtraces from more CPUs using the re-entrant > printk_printk_cpu lock. It was a preparation step for removing the obsolete > nmi_safe buffers. > > The single-line messages about idle CPUs were not serialized against other > CPUs and might appear in the middle of backtrace from another CPU, > for example: > > [56394.590068] NMI backtrace for cpu 2 > [56394.590069] CPU: 2 PID: 444 Comm: systemd-journal Not tainted 5.14.0-rc1-default+ #268 > [56394.590071] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014 > [56394.590072] RIP: 0010:lock_is_held_type+0x0/0x120 > [56394.590071] NMI backtrace for cpu 0 skipped: idling at native_safe_halt+0xb/0x10 > [56394.590076] Code: a2 38 ff 0f 0b 8b 44 24 04 eb bd 48 8d ... > [56394.590077] RSP: 0018:ffffab02c07c7e68 EFLAGS: 00000246 > [56394.590079] RAX: 0000000000000000 RBX: ffff9a7bc0ec8a40 RCX: ffffffffaab8eb40 > > It might cause confusion what CPU the following lines belongs to and > whether the backtraces are really serialized.
I originally implemented this, but later decided against it because it causes idle CPUs to begin busy-waiting in NMI context in order to log a single line saying they are idle. If the user is aware that there is only 1 line for the idle message, then the user knows that it isn't causing a problem for reading the stack trace.
When triggering many such dumps on systems with many CPUs where this patch is applied, it seemed like I was making the whole system work awfully hard for something that should be trivial.
Considering that dump_stack() and show_regs() should be fast and we are only dumping to the lockless buffer, it is probably OK to be doing all the busy-waiting. Once atomic consoles are introduced, it will have quite an impact here, but atomic consoles are mostly only active on system crash, so I think that would be OK as well.
Feel free to add:
Reviewed-by: John Ogness <john.ogness@linutronix.de>
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