Максим, VBart спасибо за помощь!
Прочитал статью про if, почистил конфиги, core dump'ов практически нет

Появляются ( около десятка записей в день ):

pid 13382 (nginx), uid 80: exited on signal 10 (core dumped)
pid 47327 (nginx), uid 80: exited on signal 11 (core dumped)
pid 49007 (nginx), uid 80: exited on signal 11
pid 52523 (nginx), uid 80: exited on signal 11


(gdb) bt
#0 0x00000008010c8786 in memcpy () from /lib/libc.so.7
#1 0x000000000049efa4 in ngx_http_proxy_create_request (r=0x801f83b00) at src/http/modules/ngx_http_proxy_module.c:1174
#2 0x0000000000468b91 in ngx_http_upstream_init_request (r=0x801f83b00) at src/http/ngx_http_upstream.c:505
#3 0x0000000000468924 in ngx_http_upstream_init (r=0x801f83b00) at src/http/ngx_http_upstream.c:446
#4 0x000000000045eca3 in ngx_http_read_client_request_body (r=0x801f83b00, post_handler=0x468840 <ngx_http_upstream_init>) at src/http/ngx_http_request_body.c:59
#5 0x000000000049d9dc in ngx_http_proxy_handler (r=0x801f83b00) at src/http/modules/ngx_http_proxy_module.c:702
#6 0x00000000004469cd in ngx_http_core_content_phase (r=0x801f83b00, ph=0x802542fa8) at src/http/ngx_http_core_module.c:1396
#7 0x0000000000445502 in ngx_http_core_run_phases (r=0x801f83b00) at src/http/ngx_http_core_module.c:877
#8 0x000000000044547d in ngx_http_handler (r=0x801f83b00) at src/http/ngx_http_core_module.c:860
#9 0x0000000000453d94 in ngx_http_process_request (r=0x801f83b00) at src/http/ngx_http_request.c:1685
#10 0x0000000000452795 in ngx_http_process_request_headers (rev=0x806203640) at src/http/ngx_http_request.c:1135
#11 0x0000000000451f26 in ngx_http_process_request_line (rev=0x806203640) at src/http/ngx_http_request.c:933
#12 0x0000000000451128 in ngx_http_init_request (rev=0x806203640) at src/http/ngx_http_request.c:519
#13 0x000000000042d47c in ngx_event_process_posted (cycle=0x801e55050, posted=0x5f0a60) at src/event/ngx_event_posted.c:40
#14 0x000000000042af25 in ngx_process_events_and_timers (cycle=0x801e55050) at src/event/ngx_event.c:274
#15 0x0000000000438264 in ngx_worker_process_cycle (cycle=0x801e55050, data=0x0) at src/os/unix/ngx_process_cycle.c:808
#16 0x0000000000435261 in ngx_spawn_process (cycle=0x801e55050, proc=0x438070 <ngx_worker_process_cycle>, data=0x0, name=0x4c1869 "worker process", respawn=2) at src/os/unix/ngx_process.c:198
#17 0x0000000000437c92 in ngx_reap_children (cycle=0x801e55050) at src/os/unix/ngx_process_cycle.c:622
#18 0x00000000004369e6 in ngx_master_process_cycle (cycle=0x801e55050) at src/os/unix/ngx_process_cycle.c:181
#19 0x00000000004066d3 in main (argc=1, argv=0x7fffffffed68) at src/core/nginx.c:410