[RFC PATCH 0/3] Implement IRQ stack on ARM64

[Jungseok Lee]

ARM64 kernel allocates 16KB kernel stack when creating a process. In case
of low memory platforms with tough workloads on userland, this order-2
allocation request reaches to memory pressure and performance degradation
simultaenously since VM page allocator falls into slowpath frequently,
which triggers page reclaim and compaction.

I believe that one of the best solutions is to reduce kernel stack size.
According to the following data from stack tracer with some fixes, [1],
a separate IRQ stack would greatly help to decrease a kernel stack depth.

	        Depth    Size   Location    (51 entries)
	        -----    ----   --------
	  0)     5352      96   _raw_spin_unlock_irqrestore+0x1c/0x60
	  1)     5256      48   gic_raise_softirq+0xa0/0xbc
	  2)     5208      80   smp_cross_call+0x40/0xbc
	  3)     5128      48   smp_send_reschedule+0x38/0x48
	  4)     5080      32   trigger_load_balance+0x184/0x29c
	  5)     5048     112   scheduler_tick+0xac/0x104
	  6)     4936      64   update_process_times+0x5c/0x74
	  7)     4872      32   tick_sched_handle.isra.15+0x38/0x7c
	  8)     4840      48   tick_sched_timer+0x48/0x90
	  9)     4792      48   __run_hrtimer+0x60/0x258
	 10)     4744      64   hrtimer_interrupt+0xe8/0x260
	 11)     4680     128   arch_timer_handler_virt+0x38/0x48
	 12)     4552      32   handle_percpu_devid_irq+0x84/0x188
	 13)     4520      64   generic_handle_irq+0x38/0x54
	 14)     4456      32   __handle_domain_irq+0x68/0xbc
	 15)     4424      64   gic_handle_irq+0x38/0x88
	 16)     4360     280   el1_irq+0x64/0xd8
	 17)     4080     168   ftrace_ops_no_ops+0xb4/0x16c
	 18)     3912      32   ftrace_call+0x0/0x4
	 19)     3880     144   __alloc_skb+0x48/0x180
	 20)     3736      96   alloc_skb_with_frags+0x74/0x234
	 21)     3640     112   sock_alloc_send_pskb+0x1d0/0x294
	 22)     3528     160   sock_alloc_send_skb+0x44/0x54
	 23)     3368      64   __ip_append_data.isra.40+0x78c/0xb48
	 24)     3304     224   ip_append_data.part.42+0x98/0xe8
	 25)     3080     112   ip_append_data+0x68/0x7c
	 26)     2968      96   icmp_push_reply+0x7c/0x144
	 27)     2872      96   icmp_send+0x3c0/0x3c8
	 28)     2776     192   __udp4_lib_rcv+0x5b8/0x684
	 29)     2584      96   udp_rcv+0x2c/0x3c
	 30)     2488      32   ip_local_deliver+0xa0/0x224
	 31)     2456      48   ip_rcv+0x360/0x57c
	 32)     2408      64   __netif_receive_skb_core+0x4d0/0x80c
	 33)     2344     128   __netif_receive_skb+0x24/0x84
	 34)     2216      32   process_backlog+0x9c/0x15c
	 35)     2184      80   net_rx_action+0x1ec/0x32c
	 36)     2104     160   __do_softirq+0x114/0x2f0
	 37)     1944     128   do_softirq+0x60/0x68
	 38)     1816      32   __local_bh_enable_ip+0xb0/0xd4
	 39)     1784      32   ip_finish_output+0x1f4/0xabc
	 40)     1752      96   ip_output+0xf0/0x120
	 41)     1656      64   ip_local_out_sk+0x44/0x54
	 42)     1592      32   ip_send_skb+0x24/0xbc
	 43)     1560      48   udp_send_skb+0x1b4/0x2f4
	 44)     1512      80   udp_sendmsg+0x2a8/0x7a0
	 45)     1432     272   inet_sendmsg+0xa0/0xd0
	 46)     1160      48   sock_sendmsg+0x30/0x78
	 47)     1112      32   ___sys_sendmsg+0x15c/0x26c
	 48)     1080     400   __sys_sendmmsg+0x94/0x180
	 49)      680     320   SyS_sendmmsg+0x38/0x54
	 50)      360     360   el0_svc_naked+0x20/0x28

So, this patch set implements a separate percpu IRQ stack. 

AFAIK, a stack tracer on ftrace does not work well. Thus, this is a single
todo list at this moment.

This series is written on top of 4.2-rc5 with drangon410c board, and it has
been validated with two different tracks: 4.2-rc5 + Linaro Ubuntu 15.04 and
3.10 + Android.

After this merge window, I will rebase this series and resend it.

Any comments or feedbacks are always welcome.

Thanks in advance!

[1]: https://lkml.org/lkml/2015/7/13/29  

Jungseok Lee (3):
  arm64: entry: Remove unnecessary calculation for S_SP in EL1h
  arm64: Introduce IRQ stack
  arm64: Reduce kernel stack size when using IRQ stack

 arch/arm64/Kconfig.debug             | 10 ++
 arch/arm64/include/asm/irq.h         |  8 ++
 arch/arm64/include/asm/thread_info.h | 19 ++++
 arch/arm64/kernel/asm-offsets.c      |  8 ++
 arch/arm64/kernel/entry.S            | 85 +++++++++++++++-
 arch/arm64/kernel/head.S             |  7 ++
 arch/arm64/kernel/irq.c              | 18 ++++
 7 files changed, 150 insertions(+), 5 deletions(-)

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1.9.1