On Thu, Feb 18, 2021 at 10:50:53AM -0500, Mathieu Desnoyers wrote: > ----- On Feb 18, 2021, at 10:06 AM, paulmck paulmck@xxxxxxxxxx wrote: > > > On Thu, Feb 18, 2021 at 09:17:01AM -0500, Mathieu Desnoyers wrote: > >> ----- On Feb 18, 2021, at 7:52 AM, Sangmoon Kim sangmoon.kim@xxxxxxxxxxx wrote: > >> > >> >> -----Original Message----- > >> >> From: neeraju=codeaurora.org@xxxxxxxxxxxxxxxxx > >> >> <neeraju=codeaurora.org@xxxxxxxxxxxxxxxxx> > >> >> Sent: Thursday, February 18, 2021 3:18 AM > >> >> > >> >> Hi Sangmoon, > >> >> > >> >> On 2/17/2021 7:19 PM, Sangmoon Kim wrote: > >> >> >> -----Original Message----- > >> >> >> From: Paul E. McKenney <paulmck@xxxxxxxxxx> > >> >> >> Sent: Wednesday, February 17, 2021 2:50 AM > >> >> >> > >> >> >> On Mon, Feb 15, 2021 at 05:53:25PM +0900, Sangmoon Kim wrote: > >> >> >>> The event allows us to trace the RCU stall when > >> >> >>> sysctl_panic_on_rcu_stall is disabled. > >> >> >>> > >> >> >>> The first parameter is the name of RCU flavour like other trace > >> >> >>> events. The second one shows us which function detected stalls. > >> >> >>> > >> >> >>> The RCU stall is mainly caused by external factors such as interrupt > >> >> >>> handling or task scheduling or something else. Therefore, this event > >> >> >>> uses TRACE_EVENT macro, not dedicated one, so that someone interested > >> >> >>> in the RCU stall can use it without CONFIG_RCU_TRACE. > >> >> >>> > >> >> >>> Signed-off-by: Sangmoon Kim <sangmoon.kim@xxxxxxxxxxx> > >> >> >> > >> >> >> The patch looks plausible, but I have to ask... Why not instead just > >> >> >> get the existing information out of the console log? > >> >> >> > >> >> >> Thanx, Paul > >> >> > > >> >> > This can provide a trigger point for the RCU stall warning. > >> >> > If a module in the kernel wants to trace the stall for debugging purposes, > >> >> > there is a cost of continuing to parse the console log. > >> >> > This tracepoint is useful because it is hard to pay these costs > >> >> > especially on mobile devices. > >> >> > > >> >> > Thanks, > >> >> > Sangmoon > >> >> > > >> >> > >> >> So, the idea here is to register to these trace events from kernel > >> >> module and use that for debugging? Just curious what debugging action > >> >> module does on these traces, as they have limited information > >> >> about the stall, compared to console stall warnings, which gives a > >> >> much more detailed information about stall. > >> >> > >> >> > >> >> Thanks > >> >> Neeraj > >> > > >> > Hi Neeraj, > >> > > >> > Yes, a module can log the stall occurence using the trace, although > >> > there is no detailed information. If the kernel panic occurs for some > >> > reasons, the debugging report generated by the module can include that > >> > RCU stall warning has occurred before. > >> > > >> > In addition, it's just an idea now, when a trace event happens, the > >> > module can store the console log including detailed information, or may > >> > also obtain CPU/task information by parsing the console log. > >> > >> Adding a new tracepoint is not just about what is extracted by this specific > >> tracepoint, but rather how it can be analyzed when combined with all other > >> relevant > >> tracepoints. > >> > >> For instance, if we have this added RCU stall warning added, here is how it can > >> be > >> used with the upcoming LTTng 2.13, which implements the "event notification" > >> triggers > >> feature: > >> > >> 1) Setup "flight recorder" (snapshot) tracing to trace into a circular ring > >> buffer, > >> enabling the following tracepoints: > >> - kernel activity (meaning all other RCU event, scheduling, irq, workqueues, > >> ...), > >> - this new RCU stall warning event. > >> > >> 2) Add a "callstack-kernel" context to the RCU stall warning event. This will > >> sample > >> the kernel stack when the event is hit. This will provide information similar to > >> the stack trace gathered into the console log on OOPS. > >> > >> 3) Enable a trigger waiting on the RCU stall warning tracepoint to be hit. On > >> this > >> trigger, actions can be associated, such as capturing a snapshot or waking up > >> an external user-space process to perform specific actions. > >> > >> So you end up with a snapshot containing the sequence of events leading to the > >> RCU stall warning, with a kernel stack trace of the context causing the stall > >> warning to be emitted. > >> > >> I would argue that this information is more complete than just the stack trace > >> extracted through the console log. > > > > I am not so sure about that. RCU CPU stall warnings dump quite a bit more > > than a stack trace to the console. Which is why I am concerned about the > > proverbial camel's nose in the tent. ;-) > > > > So Sangmoon, what is it that you really need for this to be useful to you? > > > > Or am I missing your point? (Either Mathieu's or Sangmoon's.) > > Well there is a tracepoint to dump the console's content into the tracing buffers > as well, so technically this existing RCU stall warning information could be extracted > into a trace as well. > > AFAIU, what the new "rcu stall warning" tracepoint provides is an easy way to hook on a specific > event to trigger trace capture, without requiring to parse the console log continuously, > and a way to know when the stall warning happens in time within the trace time-line. > > That being said, there may be use-cases for extracting more details about the RCU stall > as tracepoint event fields to make it more convenient, but it does not appear to be > necessary considered that the console can be saved into trace buffers as well. > Could you please clarify how the kernel ring-buffer can also be routed over trace buffer? Probably i am not aware of if it is possible on latest kernel. -- Vlad Rezki
