linux-kernel - Re: [PATCH v3 4/7] tracing: Add conditional snapshot

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Date:   Fri, 10 Aug 2018 09:05:50 -0500
From:   Tom Zanussi <zanussi@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, mhiramat@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel@...lfernandes.org, mathieu.desnoyers@...icios.com,
        julia@...com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH v3 4/7] tracing: Add conditional snapshot

On Fri, 2018-08-10 at 16:02 +0900, Namhyung Kim wrote:
> On Thu, Aug 09, 2018 at 09:34:14AM -0500, Tom Zanussi wrote:
> > From: Tom Zanussi <tom.zanussi@...ux.intel.com>
> > 
> > Currently, tracing snapshots are context-free - they capture the
> > ring
> > buffer contents at the time the tracing_snapshot() function was
> > invoked, and nothing else.  Additionally, they're always taken
> > unconditionally - the calling code can decide whether or not to
> > take a
> > snapshot, but the data used to make that decision is kept
> > separately
> > from the snapshot itself.
> > 
> > This change adds the ability to associate with each trace instance
> > some user data, along with an 'update' function that can use that
> > data
> > to determine whether or not to actually take a snapshot.  The
> > update
> > function can then update that data along with any other state (as
> > part
> > of the data presumably), if warranted.
> > 
> > Because snapshots are 'global' per-instance, only one user can
> > enable
> > and use a conditional snapshot for any given trace instance.  To
> > enable a conditional snapshot (see details in the function and data
> > structure comments), the user calls tracing_snapshot_cond_enable().
> > Similarly, to disable a conditional snapshot and free it up for
> > other
> > users, tracing_snapshot_cond_disable() should be called.
> > 
> > To actually initiate a conditional snapshot,
> > tracing_snapshot_cond()
> > should be called.  tracing_snapshot_cond() will invoke the update()
> > callback, allowing the user to decide whether or not to actually
> > take
> > the snapshot and update the user-defined data associated with the
> > snapshot.  If the callback returns 'true', tracing_snapshot_cond()
> > will then actually take the snapshot and return.
> > 
> > This scheme allows for flexibility in snapshot implementations -
> > for
> > example, by implementing slightly different update() callbacks,
> > snapshots can be taken in situations where the user is only
> > interested
> > in taking a snapshot when a new maximum in hit versus when a value
> > changes in any way at all.  Future patches will demonstrate both
> > cases.
> > 
> > Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>
> > ---
> 
> [SNIP]
> > @@ -1741,9 +1788,16 @@ static inline bool
> > event_command_needs_rec(struct event_command *cmd_ops)
> >  	return cmd_ops->flags & EVENT_CMD_FL_NEEDS_REC;
> >  }
> >  
> > +typedef bool (*cond_update)(struct trace_array *tr, void
> > *cond_data);
> > +
> 
> I think it's better having "fn_t" suffix like 'cond_update_fn_t'.
> 

Yeah, makes sense, will change.

Thanks,

Tom