Messages in this thread | | | Date | Wed, 28 Jul 2021 10:21:29 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [PATCH] lib/nmi_backtrace: Serialize even messages about idle CPUs |
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On (21/07/27 17:53), John Ogness wrote: > 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.
I agree, but don't have any strong opinion against the patch.
> Feel free to add: > > Reviewed-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
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