This enhancement idea is a close cousin to bug #2645. I would like to have a way to list several alternatives for a probe point, and have the translator pick exactly one for implementation. With this, one could specify a preferred probe point, with an alternate probe point if the first is not available. My syntax suggestion is to use the boolean-OR ('||') operator when defining a probepoint, and the translator will resolve in the typical left-to-right, short-circuit fashion. For example, given 'probe kernel.function("foo") || kernel.function("bar") {}', the translator will resolve as follows: probeable: | probe foo | bar | used ------------------- Y | Y | foo Y | N | foo N | Y | bar N | N | *error* This sort of feature will be useful for defining portable tapsets and scripts. Some example usage ideas: probe scheduler.context_switch = kernel.mark("context_switch") || kernel.function("__switch_to") || kernel.inline("context_switch") {} If markers are available, use that. Otherwise, try __switch_to, which has good parameter information. Finally, try context_switch, but parameters might not work in inlines. Note that on some kernels all three might be ok, but only *one* is used. probe scsi.ioentry = kernel.function("scsi_prep_fn") || module("scsi_mod").function("scsi_prep_fn") {} This explicitly declares how to search for the function "scsi_prep_fn". The same can almost be achieved using the '?' operator on both alternatives, except that this new methods *requires* that one resolves correctly. (Consider the use of "scsi.*" -- if using '?' the ioentry might silently disappear on failure.) probe event.foobar = kernel.function("foobar") || foo_and_bar {} probe foo_and_bar = kernel.function("foo"), kernel.function("bar") {} This demonstrates how to achieve grouping. The translator will generate a single probe on "foobar" if it exists, or else it will use two probes on "foo" and "bar". We'll have to consider operator precedence and semantics. For example, what should happen in the following cases? 1. probe foobar = foo ? || bar {} 2. probe foobar = foo || bar ? {}
It is an interesting idea. One thing to consider though is that the different probe points are unlikely to be completely interchangeable. Specifically $target variables available at each variant may be quite different, requiring different handlers. So how about associating the handlers with the alternatives? probe kernel.mark("foo") { $arg1 ; $arg2 } || kernel.function("bar") { $var1 ; $var2 } or even with less punctuation: probe kernel.mark(....) { } else kernel.function(....) { } Another issue to consider is what to do with aliases.
(In reply to comment #1) > One thing to consider though is that the different probe points are unlikely > to be completely interchangeable. Specifically $target variables available at > each variant may be quite different, requiring different handlers. True, but aliases come to the rescue in that case. > So how about associating the handlers with the alternatives? > > probe kernel.mark("foo") { $arg1 ; $arg2 } || > kernel.function("bar") { $var1 ; $var2 } Aliases give you a way to split the necessary parts, and still share a common body: probe my_foo || my_bar { printf("this is important: %d %s\n", var1, var2) } probe my_foo = kernel.mark("foo") { var1=$arg1->var1 ; var2=$arg2->var2 } probe my_bar = kernel.function("bar") { var1=$var1 ; var2=$var2 } > or even with less punctuation: > > probe kernel.mark(....) { } > else kernel.function(....) { } Hmm... you've turned '||' into 'else'? I suppose if you prefer using keywords over punctuation, that would be fine. I'm not sure that 'else' makes sense if there's more that two alternatives though -- perhaps 'or' as a keyword? > Another issue to consider is what to do with aliases. Well, using this for tapset aliases was exactly what I intended, so lets hash it out. It seems consistent to just expand aliases as normal. probe a = b || c {} probe b = w || x {} probe c = y, z {} An instance of alias 'a' would expand to the probepoints 'w' OR 'x' OR ('y' AND 'z'). The comma is effectively 'AND', and the alias levels act as grouping for operator precedence. The precedence can get tricky though -- consider: probe a, b || c {} If the comma is just 'AND' then traditionally this would be read '(a AND b) OR c', but visually the comma seems like a lower precedence, which would make it 'a AND (b OR c)'. I'm still uncertain about how the 'optional' flags should behave in this boolean-like probe resolution...
*** Bug 2067 has been marked as a duplicate of this bug. ***
patch committed
BTW, the commited syntax is a variant of the optional flag. I realized that "||" is really a special case of "? and stop if resolved". So now we have probes like: probe kernel.function("favorite") !, kernel.function("second_best") !, kernel.function("worst, but must exist") { }