[PATCH] perf tools: allow overriding MAX_NR_CPUS at compile time

[Christophe Leroy]

After update of kernel, perf tool doesn't run anymore on my
32MB RAM powerpc board, but still runs on a 128MB RAM board:

~# strace perf
execve("/usr/sbin/perf", ["perf"], [/* 12 vars */]) = -1 ENOMEM (Cannot allocate memory)
--- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, si_addr=0} ---
+++ killed by SIGSEGV +++
Segmentation fault

objdump -x shows that .bss section has a huge size of 24Mbytes:

 27 .bss          016baca8  101cebb8  101cebb8  001cd988  2**3

With especially the following objects having quite big size

10205f80 l     O .bss	00140000              runtime_cycles_stats
10345f80 l     O .bss	00140000              runtime_stalled_cycles_front_stats
10485f80 l     O .bss	00140000              runtime_stalled_cycles_back_stats
105c5f80 l     O .bss	00140000              runtime_branches_stats
10705f80 l     O .bss	00140000              runtime_cacherefs_stats
10845f80 l     O .bss	00140000              runtime_l1_dcache_stats
10985f80 l     O .bss	00140000              runtime_l1_icache_stats
10ac5f80 l     O .bss	00140000              runtime_ll_cache_stats
10c05f80 l     O .bss	00140000              runtime_itlb_cache_stats
10d45f80 l     O .bss	00140000              runtime_dtlb_cache_stats
10e85f80 l     O .bss	00140000              runtime_cycles_in_tx_stats
10fc5f80 l     O .bss	00140000              runtime_transaction_stats
11105f80 l     O .bss	00140000              runtime_elision_stats
11245f80 l     O .bss	00140000              runtime_topdown_total_slots
11385f80 l     O .bss	00140000              runtime_topdown_slots_retired
114c5f80 l     O .bss	00140000              runtime_topdown_slots_issued
11605f80 l     O .bss	00140000              runtime_topdown_fetch_bubbles
11745f80 l     O .bss	00140000              runtime_topdown_recovery_bubbles

This is due to commit 4d255766d28b1 ("perf: Bump max number of cpus
to 1024"), because many tables are sized with MAX_NR_CPUS

This patch gives the opportunity to redefine MAX_NR_CPUS via

make EXTRA_CFLAGS=-DMAX_NR_CPUS=1

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
---
 tools/perf/perf.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/perf/perf.h b/tools/perf/perf.h
index dc442ba21bf6..a9db563da0a9 100644
--- a/tools/perf/perf.h
+++ b/tools/perf/perf.h
@@ -23,7 +23,9 @@ static inline unsigned long long rdclock(void)
 	return ts.tv_sec * 1000000000ULL + ts.tv_nsec;
 }
 
+#ifndef MAX_NR_CPUS
 #define MAX_NR_CPUS			1024
+#endif
 
 extern const char *input_name;
 extern bool perf_host, perf_guest;
-- 
2.13.3