Messages in this thread | | | Subject | Re: [PATCH 1/2 v2] tracing/events: provide string with undefined size support | From | Peter Zijlstra <> | Date | Fri, 17 Apr 2009 08:22:40 +0200 |
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On Thu, 2009-04-16 at 22:28 +0200, Frédéric Weisbecker wrote: > 2009/4/16 Peter Zijlstra <a.p.zijlstra@chello.nl>: > > On Thu, 2009-04-16 at 22:00 +0200, Frederic Weisbecker wrote: > >> Impact: less memory usage for tracing > >> > >> This patch provides the support for dynamic size strings on > >> event tracing. > >> > >> The key concept is to use a structure with an ending char array field of > >> undefined size and use such ability to allocate the minimal size on the ring > >> buffer to make the entry fit inside as opposite to a fixed length strings with > >> upper bound. > >> > >> This patch provides one new macro: > >> > >> -__ending_string(name, src) > >> > >> This one declares the string to the structure inside TP_STRUCT__entry. > >> You need to provide the name of the string field and the source that will be > >> copied inside. > >> Two constraints: only one __ending_string() per TRACE_EVENT can be added and > >> it must be the last field to be declared. Hence the __ending prefix. > >> > >> This macro will declare the necessary field and will also add the dynamic > >> size of the string needed for the ring buffer entry allocation. > >> > >> It also support filtering because these strings behave essentially > >> like usual fixed length string. > > > > can't we simply do __string(name, src) and output something like: > > > > struct { > > u16 size; > > char str[0]; > > } name; > > > > That would get rid of this __ending_ wart. > > > > > > Hmm, I don't understand. > Such a thing doesn't seem to work. Once we fill the string it would > override the fields that > follow it if it's not at the end.
Just grow the thing to fit whatever string length -- rather common pattern:
struct foo { int length; char data[0]; };
struct foo *bar = kmalloc(sizeof(struct foo) + data_size);
and bob's your uncle.
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