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The idea here is to avoid a memory access and conditional branch per
probe site. Instead, the probe is represented by an "unreachable"
unconditional function call. asm goto is used to store the address of
the probe site (represented by a no-op sled) and the address of the
function call into a tracepoint record. Each SDT probe carries a list
of tracepoints.
When the probe is enabled, the no-op sled corresponding to each
tracepoint is overwritten with a jmp to the corresponding label. The
implementation uses smp_rendezvous() to park all other CPUs while the
instruction is being overwritten, as this can't be done atomically in
general. The compiler moves argument marshalling code and the
sdt_probe() function call out-of-line, i.e., to the end of the function.
Per gallatin@ in D43504, this approach has less overhead when probes are
disabled. To make the implementation a bit simpler, I removed support
for probes with 7 arguments; nothing makes use of this except a
regression test case. It could be re-added later if need be.
The approach taken in this patch enables some more improvements:
1. We can now automatically fill out the "function" field of SDT probe
names. The SDT macros let the programmer specify the function and
module names, but this is really a bug and shouldn't have been
allowed. The intent was to be able to have the same probe in
multiple functions and to let the user restrict which probes actually
get enabled by specifying a function name or glob.
2. We can avoid branching on SDT_PROBES_ENABLED() by adding the ability
to include blocks of code in the out-of-line path. For example:
if (SDT_PROBES_ENABLED()) {
int reason = CLD_EXITED;
if (WCOREDUMP(signo))
reason = CLD_DUMPED;
else if (WIFSIGNALED(signo))
reason = CLD_KILLED;
SDT_PROBE1(proc, , , exit, reason);
}
could be written
SDT_PROBE1_EXT(proc, , , exit, reason,
int reason;
reason = CLD_EXITED;
if (WCOREDUMP(signo))
reason = CLD_DUMPED;
else if (WIFSIGNALED(signo))
reason = CLD_KILLED;
);
In the future I would like to use this mechanism more generally, e.g.,
to remove branches and marshalling code used by hwpmc, and generally to
make it easier to add new tracepoint consumers without having to add
more conditional branches to hot code paths.
Reviewed by: Domagoj Stolfa, avg
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D44483
109 lines
3 KiB
C
109 lines
3 KiB
C
/*-
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* Copyright 2008 John Birrell <jb@FreeBSD.org>
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*
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*/
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#include <sys/types.h>
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#include <sys/param.h>
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#include <sys/systm.h>
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#include <sys/conf.h>
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#include <sys/kernel.h>
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#include <sys/module.h>
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#include <sys/sdt.h>
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#include <sys/sysctl.h>
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#include <sys/vnode.h>
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SDT_PROVIDER_DEFINE(test);
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SDT_PROBE_DEFINE6(test, , , sdttest, "int", "int", "int", "int", "int",
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"int");
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/*
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* These are variables that the DTrace test suite references in the
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* Solaris kernel. We define them here so that the tests function
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* unaltered.
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*/
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int kmem_flags;
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typedef struct vnode vnode_t;
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vnode_t dummy;
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vnode_t *rootvp = &dummy;
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/*
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* Test SDT probes with more than 5 arguments. On amd64, such probes require
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* special handling since only the first 5 arguments will be passed to
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* dtrace_probe() in registers; the rest must be fetched off the stack.
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*/
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static int
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dtrace_test_sdttest(SYSCTL_HANDLER_ARGS)
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{
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int val, error;
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val = 0;
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error = sysctl_handle_int(oidp, &val, 0, req);
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if (error || req->newptr == NULL)
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return (error);
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else if (val == 0)
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return (0);
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SDT_PROBE6(test, , , sdttest, 1, 2, 3, 4, 5, 6);
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return (error);
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}
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static SYSCTL_NODE(_debug, OID_AUTO, dtracetest,
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CTLFLAG_RD | CTLFLAG_MPSAFE, 0,
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"");
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SYSCTL_PROC(_debug_dtracetest, OID_AUTO, sdttest,
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CTLTYPE_INT | CTLFLAG_MPSAFE | CTLFLAG_RW, NULL, 0, dtrace_test_sdttest,
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"I", "Trigger the SDT test probe");
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static int
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dtrace_test_modevent(module_t mod, int type, void *data)
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{
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int error = 0;
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switch (type) {
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case MOD_LOAD:
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break;
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case MOD_UNLOAD:
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break;
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case MOD_SHUTDOWN:
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break;
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default:
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error = EOPNOTSUPP;
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break;
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}
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return (error);
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}
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DEV_MODULE(dtrace_test, dtrace_test_modevent, NULL);
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MODULE_VERSION(dtrace_test, 1);
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MODULE_DEPEND(dtrace_test, dtraceall, 1, 1, 1);
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