LU-2622 obdclass: Remove the global cl_env list

LU-2622 obdclass: Remove the global cl_env list

New cl_env structures are allocated using a SLAB cache specifically
created for allocating and freeing these structure. Without this patch,
when a thread is finished with a cl_env structure it places it on a
global list instead of freeing it back to the SLAB cache. With this
patch, this global list is completely removed, and cl_env structures are
released immediately back to the SLAB cache.

The motivation for this change essentially boils down to this secondary
global list cache being completely unnecessary, and only serving to
serialize any calls to cl_env_get and cl_env_put. This has proven to
cause a severe performance impact on large core count systems,
specifically during memory reclamation (i.e. ll_releasepage).

For example, on BG/Q Sequoia IO nodes, we've experienced nearly all
68 cores of a machine spinning on the lock protecting this global
list. Some example stack traces showcasing this problem were gathered
using sysrq-l, and are displayed below:

    CPU56:
    Call Trace:
    [c00000000fe3bb30] [c000000000008d1c] .show_stack+0x7c/0x184 (unreliable)
    [c00000000fe3bbe0] [c00000000027604c] .showacpu+0x64/0x94
    [c00000000fe3bc70] [c000000000068b30] .generic_smp_call_function_interrupt+0x10c/0x230
    [c00000000fe3bd40] [c00000000001d11c] .smp_message_recv+0x34/0x78
    [c00000000fe3bdc0] [c00000000002526c] .bgq_ipi_dispatch+0x118/0x18c
    [c00000000fe3be50] [c00000000007b20c] .handle_IRQ_event+0x88/0x18c
    [c00000000fe3bf00] [c00000000007dc90] .handle_percpu_irq+0x8c/0x100
    [c00000000fe3bf90] [c00000000001b808] .call_handle_irq+0x1c/0x2c
    [c0000003e1c4a4c0] [c0000000000059f0] .do_IRQ+0x154/0x1e0
    [c0000003e1c4a570] [c0000000000144dc] exc_external_input_book3e+0x110/0x114
    --- Exception: 501 at ._raw_spin_lock+0xd8/0x1a8
        LR = ._raw_spin_lock+0x104/0x1a8
    [c0000003e1c4a860] [8000000000b04f38] libcfs_nidstrings+0x2acc/0xfffffffffffe5824 [libcfs] (unreliable)
    [c0000003e1c4a910] [c00000000042d4cc] ._spin_lock+0x10/0x24
    [c0000003e1c4a980] [80000000024c2f4c] .cl_env_get+0xec/0x480 [obdclass]
    [c0000003e1c4aa60] [80000000024c336c] .cl_env_nested_get+0x8c/0xf0 [obdclass]
    [c0000003e1c4aaf0] [800000000692070c] .ll_releasepage+0xbc/0x200 [lustre]
    [c0000003e1c4aba0] [c000000000094110] .try_to_release_page+0x68/0x8c
    [c0000003e1c4ac10] [c0000000000a4190] .shrink_page_list.clone.0+0x3d8/0x63c
    [c0000003e1c4adc0] [c0000000000a47d8] .shrink_inactive_list+0x3e4/0x690
    [c0000003e1c4af90] [c0000000000a4f54] .shrink_zone+0x4d0/0x4d4
    [c0000003e1c4b0c0] [c0000000000a5a68] .try_to_free_pages+0x204/0x3d0
    [c0000003e1c4b220] [c00000000009d044] .__alloc_pages_nodemask+0x460/0x738
    [c0000003e1c4b3a0] [c000000000095af4] .grab_cache_page_write_begin+0x7c/0xec
    [c0000003e1c4b450] [8000000006920964] .ll_write_begin+0x94/0x270 [lustre]
    [c0000003e1c4b520] [c0000000000968c8] .generic_file_buffered_write+0x148/0x374
    [c0000003e1c4b660] [c000000000097050] .__generic_file_aio_write+0x374/0x3d8
    [c0000003e1c4b760] [c00000000009712c] .generic_file_aio_write+0x78/0xe8
    [c0000003e1c4b810] [800000000693ed4c] .vvp_io_write_start+0xfc/0x3e0 [lustre]
    [c0000003e1c4b8e0] [80000000024d9c6c] .cl_io_start+0xcc/0x220 [obdclass]
    [c0000003e1c4b980] [80000000024e1a84] .cl_io_loop+0x194/0x2c0 [obdclass]
    [c0000003e1c4ba30] [80000000068ba1d8] .ll_file_io_generic+0x498/0x670 [lustre]
    [c0000003e1c4bb30] [80000000068ba834] .ll_file_aio_write+0x1d4/0x3a0 [lustre]
    [c0000003e1c4bc00] [80000000068bab50] .ll_file_write+0x150/0x320 [lustre]
    [c0000003e1c4bce0] [c0000000000d1ba8] .vfs_write+0xd0/0x1c4
    [c0000003e1c4bd80] [c0000000000d1d98] .SyS_write+0x54/0x98
    [c0000003e1c4be30] [c000000000000580] syscall_exit+0x0/0x2c

    CPU63:
    Call Trace:
    [c00000000fe03b30] [c000000000008d1c] .show_stack+0x7c/0x184 (unreliable)
    [c00000000fe03be0] [c00000000027604c] .showacpu+0x64/0x94
    [c00000000fe03c70] [c000000000068b30] .generic_smp_call_function_interrupt+0x10c/0x230
    [c00000000fe03d40] [c00000000001d11c] .smp_message_recv+0x34/0x78
    [c00000000fe03dc0] [c00000000002526c] .bgq_ipi_dispatch+0x118/0x18c
    [c00000000fe03e50] [c00000000007b20c] .handle_IRQ_event+0x88/0x18c
    [c00000000fe03f00] [c00000000007dc90] .handle_percpu_irq+0x8c/0x100
    [c00000000fe03f90] [c00000000001b808] .call_handle_irq+0x1c/0x2c
    [c0000003c4f0a510] [c0000000000059f0] .do_IRQ+0x154/0x1e0
    [c0000003c4f0a5c0] [c0000000000144dc] exc_external_input_book3e+0x110/0x114
    --- Exception: 501 at ._raw_spin_lock+0xdc/0x1a8
        LR = ._raw_spin_lock+0x104/0x1a8
    [c0000003c4f0a8b0] [800000000697a578] msgdata.87439+0x20/0xfffffffffffccf88 [lustre] (unreliable)
    [c0000003c4f0a960] [c00000000042d4cc] ._spin_lock+0x10/0x24
    [c0000003c4f0a9d0] [80000000024c17e8] .cl_env_put+0x178/0x420 [obdclass]
    [c0000003c4f0aa70] [80000000024c1ab0] .cl_env_nested_put+0x20/0x40 [obdclass]
    [c0000003c4f0aaf0] [8000000006920794] .ll_releasepage+0x144/0x200 [lustre]
    [c0000003c4f0aba0] [c000000000094110] .try_to_release_page+0x68/0x8c
    [c0000003c4f0ac10] [c0000000000a4190] .shrink_page_list.clone.0+0x3d8/0x63c
    [c0000003c4f0adc0] [c0000000000a47d8] .shrink_inactive_list+0x3e4/0x690
    [c0000003c4f0af90] [c0000000000a4f54] .shrink_zone+0x4d0/0x4d4
    [c0000003c4f0b0c0] [c0000000000a5a68] .try_to_free_pages+0x204/0x3d0
    [c0000003c4f0b220] [c00000000009d044] .__alloc_pages_nodemask+0x460/0x738
    [c0000003c4f0b3a0] [c000000000095af4] .grab_cache_page_write_begin+0x7c/0xec
    [c0000003c4f0b450] [8000000006920964] .ll_write_begin+0x94/0x270 [lustre]
    [c0000003c4f0b520] [c0000000000968c8] .generic_file_buffered_write+0x148/0x374
    [c0000003c4f0b660] [c000000000097050] .__generic_file_aio_write+0x374/0x3d8
    [c0000003c4f0b760] [c00000000009712c] .generic_file_aio_write+0x78/0xe8
    [c0000003c4f0b810] [800000000693ed4c] .vvp_io_write_start+0xfc/0x3e0 [lustre]
    [c0000003c4f0b8e0] [80000000024d9c6c] .cl_io_start+0xcc/0x220 [obdclass]
    [c0000003c4f0b980] [80000000024e1a84] .cl_io_loop+0x194/0x2c0 [obdclass]
    [c0000003c4f0ba30] [80000000068ba1d8] .ll_file_io_generic+0x498/0x670 [lustre]
    [c0000003c4f0bb30] [80000000068ba834] .ll_file_aio_write+0x1d4/0x3a0 [lustre]
    [c0000003c4f0bc00] [80000000068bab50] .ll_file_write+0x150/0x320 [lustre]
    [c0000003c4f0bce0] [c0000000000d1ba8] .vfs_write+0xd0/0x1c4
    [c0000003c4f0bd80] [c0000000000d1d98] .SyS_write+0x54/0x98
    [c0000003c4f0be30] [c000000000000580] syscall_exit+0x0/0x2c

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: Ief4b524784e07d7677ecb8a9ce97a7b54ccc6f75
Reviewed-on: http://review.whamcloud.com/5204
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>