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SubjectRe: [patch 1/2] Linux Kernel Markers - Support Multiple Probes
On Tue, 04 Dec 2007 13:18:46 -0500
Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:

> RCU style multiple probes support for the Linux Kernel Markers.
> Common case (one probe) is still fast and does not require dynamic allocation
> or a supplementary pointer dereference on the fast path.
>
> - Move preempt disable from the marker site to the callback.
>
> Since we now have an internal callback, move the preempt disable/enable to the
> callback instead of the marker site.
>
> Since the callback change is done asynchronously (passing from a handler that
> supports arguments to a handler that does not setup the arguments is no
> arguments are passed), we can safely update it even if it is outside the preempt
> disable section.
>
> - Move probe arm to probe connection. Now, a connected probe is automatically
> armed.
>
> Remove MARK_MAX_FORMAT_LEN, unused.
>
> This patch modifies the Linux Kernel Markers API : it removes the probe
> "arm/disarm" and changes the probe function prototype : it now expects a
> va_list * instead of a "...".
>
> If we want to have more than one probe connected to a marker at a given
> time (LTTng, or blktrace, ssytemtap) then we need this patch. Without it,
> connecting a second probe handler to a marker will fail.
>
> It allow us, for instance, to do interesting combinations :
>
> Do standard tracing with LTTng and, eventually, to compute statistics
> with SystemTAP, or to have a special trigger on an event that would call
> a systemtap script which would stop flight recorder tracing.
>
> ...
>
> +/*
> + * Note about RCU :

Paul cc'ed in case he has time to review this work...

> + * It is used to make sure every handler has finished using its private data
> + * between two consecutive operation (add or remove) on a given marker. It is
> + * also used to delay the free of multiple probes array until a quiescent state
> + * is reached.
> + */
> struct marker_entry {
> struct hlist_node hlist;
> char *format;
> - marker_probe_func *probe;
> - void *private;
> + void (*call)(const struct marker *mdata, /* Probe wrapper */
> + void *call_private, const char *fmt, ...);
> + struct marker_probe_closure single;
> + struct marker_probe_closure *multi;
> int refcount; /* Number of times armed. 0 if disarmed. */
> + struct rcu_head rcu;
> + void *oldptr;
> + char rcu_pending:1;
> + char ptype:1;

rcu_pending and ptype share the same word and modifications of one can
trash modifications of the other on a different cpu. External locking is
needed to prevent this. Is it present? If so, it should be documented
right here in a comment. If not, use plain-old-ints.


> char name[0]; /* Contains name'\0'format'\0' */
> };
>
> @@ -63,7 +69,8 @@ static struct hlist_head marker_table[MA
>
> /**
> * __mark_empty_function - Empty probe callback
> - * @mdata: pointer of type const struct marker
> + * @probe_private: probe private data
> + * @call_private: call site private data
> * @fmt: format string
> * @...: variable argument list
> *
> @@ -72,13 +79,262 @@ static struct hlist_head marker_table[MA
> * though the function pointer change and the marker enabling are two distinct
> * operations that modifies the execution flow of preemptible code.
> */
> -void __mark_empty_function(const struct marker *mdata, void *private,
> - const char *fmt, ...)
> +void __mark_empty_function(void *probe_private, void *call_private,
> + const char *fmt, va_list *args)
> {
> }
> EXPORT_SYMBOL_GPL(__mark_empty_function);
>
> /*
> + * marker_probe_cb Callback that prepares the variable argument list for probes.
> + * @mdata: pointer of type struct marker
> + * @call_private: caller site private data
> + * @fmt: format string
> + * @...: Variable argument list.
> + *
> + * Since we do not use "typical" pointer based RCU in the 1 argument case, we
> + * need to put a full smp_rmb() in this branch. This is why we do not use
> + * rcu_dereference() for the pointer read.

hm.

> + */
> +void marker_probe_cb(const struct marker *mdata, void *call_private,
> + const char *fmt, ...)
> +{
> + va_list args;
> + char ptype;
> +
> + preempt_disable();

What are the preempt_disable()s doing in here?

Unless I missed something obvious, a comment is needed here (at least).

> + ptype = ACCESS_ONCE(mdata->ptype);
> + if (likely(!ptype)) {
> + marker_probe_func *func;
> + /* Must read the ptype before ptr. They are not data dependant,
> + * so we put an explicit smp_rmb() here. */
> + smp_rmb();
> + func = ACCESS_ONCE(mdata->single.func);
> + /* Must read the ptr before private data. They are not data
> + * dependant, so we put an explicit smp_rmb() here. */
> + smp_rmb();
> + va_start(args, fmt);
> + func(mdata->single.probe_private, call_private, fmt, &args);
> + va_end(args);
> + } else {
> + struct marker_probe_closure *multi;
> + int i;
> + /*
> + * multi points to an array, therefore accessing the array
> + * depends on reading multi. However, even in this case,
> + * we must insure that the pointer is read _before_ the array
> + * data. Same as rcu_dereference, but we need a full smp_rmb()
> + * in the fast path, so put the explicit barrier here.
> + */
> + smp_read_barrier_depends();
> + multi = ACCESS_ONCE(mdata->multi);
> + for (i = 0; multi[i].func; i++) {
> + va_start(args, fmt);
> + multi[i].func(multi[i].probe_private, call_private, fmt,
> + &args);
> + va_end(args);
> + }
> + }
> + preempt_enable();
> +}
> +EXPORT_SYMBOL_GPL(marker_probe_cb);
>
> ...
>
> +static inline void debug_print_probes(struct marker_entry *entry)
> +{
> + int i;
> +
> + if (!marker_debug)
> + return;
> +
> + if (!entry->ptype) {
> + printk(KERN_DEBUG "Single probe : %p %p\n",
> + entry->single.func,
> + entry->single.probe_private);
> + } else {
> + for (i = 0; entry->multi[i].func; i++)
> + printk(KERN_DEBUG "Multi probe %d : %p %p\n", i,
> + entry->multi[i].func,
> + entry->multi[i].probe_private);
> + }
> +}

argh, this has about six callsites. It is vastly too large to inline.

> +static struct marker_probe_closure *
> +marker_entry_add_probe(struct marker_entry *entry,
> + marker_probe_func *probe, void *probe_private)
> +{
> + int nr_probes = 0;
> + struct marker_probe_closure *old, *new;
> +
> + WARN_ON(!probe);
> +
> + debug_print_probes(entry);
> + old = entry->multi;
> + if (!entry->ptype) {
> + if (entry->single.func == probe &&
> + entry->single.probe_private == probe_private)
> + return ERR_PTR(-EBUSY);
> + if (entry->single.func == __mark_empty_function) {
> + /* 0 -> 1 probes */
> + entry->single.func = probe;
> + entry->single.probe_private = probe_private;
> + entry->refcount = 1;
> + entry->ptype = 0;
> + debug_print_probes(entry);
> + return NULL;
> + } else {
> + /* 1 -> 2 probes */
> + nr_probes = 1;
> + old = NULL;
> + }
> + } else {
> + /* (N -> N+1), (N != 0, 1) probes */
> + for (nr_probes = 0; old[nr_probes].func; nr_probes++)
> + if (old[nr_probes].func == probe
> + && old[nr_probes].probe_private
> + == probe_private)
> + return ERR_PTR(-EBUSY);
> + }
> + /* + 2 : one for new probe, one for NULL func */
> + new = kzalloc((nr_probes + 2) * sizeof(struct marker_probe_closure),
> + GFP_KERNEL);
> + if (new == NULL)
> + return ERR_PTR(-ENOMEM);
> + if (!old)
> + new[0] = entry->single;
> + else
> + memcpy(new, old,
> + nr_probes * sizeof(struct marker_probe_closure));

could use krealloc here, although it isn't a very good fit.

> + new[nr_probes].func = probe;
> + new[nr_probes].probe_private = probe_private;
> + entry->refcount = nr_probes + 1;
> + entry->multi = new;
> + entry->ptype = 1;
> + debug_print_probes(entry);
> + return old;
> +}
> +



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