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SubjectRe: [RFC PATCH 1/9] cxl/mem: Implement Get Event Records command
On Tue, 20 Sep 2022 15:10:26 -0700
Ira Weiny <ira.weiny@intel.com> wrote:

> On Tue, Sep 20, 2022 at 01:23:29PM -0700, Jiang, Dave wrote:
> >
> > On 9/20/2022 8:49 AM, Jonathan Cameron wrote:
> > > On Fri, 9 Sep 2022 13:53:55 -0700
> > > Ira Weiny <ira.weiny@intel.com> wrote:
> > >
> > > > On Thu, Sep 08, 2022 at 01:52:40PM +0100, Jonathan Cameron wrote:
> > > > [snip]
> > > >
> > > > > > > > diff --git a/include/trace/events/cxl-events.h b/include/trace/events/cxl-events.h
> > > > > > > > new file mode 100644
> > > > > > > > index 000000000000..f4baeae66cf3
> > > > > > > > --- /dev/null
> > > > > > > > +++ b/include/trace/events/cxl-events.h
> > > > > > > > @@ -0,0 +1,127 @@
> > > > > > > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > > > > > > +#undef TRACE_SYSTEM
> > > > > > > > +#define TRACE_SYSTEM cxl_events
> > > > > > > > +
> > > > > > > > +#if !defined(_CXL_TRACE_EVENTS_H) || defined(TRACE_HEADER_MULTI_READ)
> > > > > > > > +#define _CXL_TRACE_EVENTS_H
> > > > > > > > +
> > > > > > > > +#include <linux/tracepoint.h>
> > > > > > > > +
> > > > > > > > +#define EVENT_LOGS \
> > > > > > > > + EM(CXL_EVENT_TYPE_INFO, "Info") \
> > > > > > > > + EM(CXL_EVENT_TYPE_WARN, "Warning") \
> > > > > > > > + EM(CXL_EVENT_TYPE_FAIL, "Failure") \
> > > > > > > > + EM(CXL_EVENT_TYPE_FATAL, "Fatal") \
> > > > > > > > + EMe(CXL_EVENT_TYPE_MAX, "<undefined>")
> > > > > > > Hmm. 4 is defined in CXL 3.0, but I'd assume we won't use tracepoints for
> > > > > > > dynamic capacity events so I guess it doesn't matter.
> > > > > > I'm not sure why you would say that. I anticipate some user space daemon
> > > > > > requiring these events to set things up.
> > > > > Certainly a possible solution. I'd kind of expect a more hand shake based approach
> > > > > than a tracepoint. Guess we'll see :)
> > > > Yea I think we should wait an see.
> > > >
> > > > > > > > + { CXL_EVENT_RECORD_FLAG_PERF_DEGRADED, "Performance Degraded" }, \
> > > > > > > > + { CXL_EVENT_RECORD_FLAG_HW_REPLACE, "Hardware Replacement Needed" } \
> > > > > > > > +)
> > > > > > > > +
> > > > > > > > +TRACE_EVENT(cxl_event,
> > > > > > > > +
> > > > > > > > + TP_PROTO(const char *dev_name, enum cxl_event_log_type log,
> > > > > > > > + struct cxl_event_record_raw *rec),
> > > > > > > > +
> > > > > > > > + TP_ARGS(dev_name, log, rec),
> > > > > > > > +
> > > > > > > > + TP_STRUCT__entry(
> > > > > > > > + __string(dev_name, dev_name)
> > > > > > > > + __field(int, log)
> > > > > > > > + __array(u8, id, UUID_SIZE)
> > > > > > > > + __field(u32, flags)
> > > > > > > > + __field(u16, handle)
> > > > > > > > + __field(u16, related_handle)
> > > > > > > > + __field(u64, timestamp)
> > > > > > > > + __array(u8, data, EVENT_RECORD_DATA_LENGTH)
> > > > > > > > + __field(u8, length)
> > > > > > > Do we want the maintenance operation class added in Table 8-42 from CXL 3.0?
> > > > > > > (only noticed because I happen to have that spec revision open rather than 2.0).
> > > > > > Yes done.
> > > > > >
> > > > > > There is some discussion with Dan regarding not decoding anything and letting
> > > > > > user space take care of it all. I think this shows a valid reason Dan
> > > > > > suggested this.
> > > > > I like being able to print tracepoints with out userspace tools.
> > > > > This also enforces structure and stability of interface which I like.
> > > > I tend to agree with you.
> > > >
> > > > > Maybe a raw tracepoint or variable length trailing buffer to pass
> > > > > on what we don't understand?
> > > > I've already realized that we need to print all reserved fields for this
> > > > reason. If there is something the kernel does not understand user space can
> > > > just figure it out on it's own.
> > > >
> > > > Sound reasonable?
> > > Hmm. Printing reserved fields would be unusual. Not sure what is done for similar
> > > cases elsewhere, CPER records etc...
> > >
> > > We could just print a raw array of the whole event as well as decode version, but
> > > that means logging most of the fields twice...
> > >
> > > Not nice either.
> > >
> > > I'm a bit inclined to say we should maybe just ignore stuff we don't know about or
> > > is there a version number we can use to decide between decoded vs decoded as much as
> > > possible + raw log?
>
> I'm not a fan of loging the raw + decoded versions.
>
> >
> > libtraceevent can pull the trace event data structure fields directly. So
> > the raw data can be pulled directly from the kernel.
>
> This raw data needs to be in a field though. If the kernel does not save the
> reserved fields in the TP_fast_assign() then the data won't be in a field to
> access.
>
> >
> > And what gets printed
> > to the trace buffer can be decoded data constructed from those fields by the
> > kernel code. So with that you can have access both.
> >
>
> Fast assigning the entire buffer + decoded versions will roughly double the
> trace event size.
>
> Thinking through this a bit more there is a sticking point.
>
> The difficulty will be ensuring that any new field names are documented such
> that when user space starts to look at them they can determine if that data
> appears as a new field or as part of a reserved field.
>
> For example if user space needs to access data in the reserved data now it can
> simply decode it. However, when that data becomes a field it no longer is part
> of the reserved data. So what user space would need to do is look for the
> field first (ie know the field name) and then if it does not appear extract it
> from the reserved data.
>
> I'm now wondering if I've wasted my time decoding anything since the kernel
> does not need to know anything about these fields. Because the above scenario
> means that user space may get ugly over time.
>
> That said I don't think it will present any incompatibilities. So perhaps we
> are ok?

I favor decoding current record in kernel and packing it appropriately.
If that means we don't provide some new data from a future version then such
is life - the kernel needs upgrading. That information is unlikely to be
crucial - it's probably just more detail.

Jonathan

>
> Ira

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