Re: kernel 5.10.x Reserves much more RAM than 5.11.0, 8 MB vs 20 MB

[Joakim Tjernlund <Joakim.Tjernlund@infinera.com>]

On Wed, 2021-10-27 at 10:12 -0700, Yang Shi wrote:
> On Wed, Oct 27, 2021 at 5:48 AM Joakim Tjernlund
> <Joakim.Tjernlund@infinera.com> wrote:
> > 
> > Using a fairly minimal kernel I get a big difference in Reserved memory, using:
> >     qemu-system-aarch64 -machine virt -kernel arch/arm64/boot/Image -cpu cortex-a53  -nographic -m 32M
> > 
> > I get this:
> > Linux version 5.11.0
> > Memory: 24052K/32768K available (3520K kernel code, 684K rwdata, 548K rodata, 320K init, 245K bss, 8716K reserved, 0K cma-reserved)
> > 
> > Versus:
> > 
> > Linux version 5.10.75
> > Memory: 11836K/32768K available (3518K kernel code, 684K rwdata, 540K rodata, 320K init, 244K bss, 20932K reserved, 0K cma-reserved)
> > 
> > Reserving 20 MB RAM on a 32 MB is way too much, even 8Mb is a bit much for us.
> 
> This might be caused by the new lockless printk buffer patch
> introduced in 5.10. We noticed this spike on 5.10 kernel too. It may
> have much more memory reserved for meta data and the total amount may
> be as much as 3x of log_buf_len. The amount varies on different
> platforms with different configs. I didn't dig too much into the
> details to figure out how it gets the number.
> 
> You could try to adjust CONFIG_LOG_BUF_SHIFT or set log_buf_len from
> kernel commandline to limit the printk buffer size.

I did try this by changing LOG_BUF_SHIFT but the resulting change was just minor.

To get some info about meminfo/zoneinfo I booted with an inird FS and got this:
  Memory: 9576K/32768K available (3518K kernel code, 460K rwdata, 540K rodata, 320K init, 180K bss, 23192K reserved, 0K cma-reserved)
The initrd only added some 3 MB's
Here are meminfo/zoneinfo, not sure if they are useful though.

 cat /proc/meminfo 
MemTotal:          12416 kB
MemFree:            4644 kB
MemAvailable:       4064 kB
Buffers:               0 kB
Cached:             4788 kB
SwapCached:            0 kB
Active:             2500 kB
Inactive:           2524 kB
Active(anon):       2500 kB
Inactive(anon):     2524 kB
Active(file):          0 kB
Inactive(file):        0 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:           248 kB
Mapped:             1844 kB
Shmem:              4788 kB
KReclaimable:          0 kB
Slab:               1712 kB
SReclaimable:          0 kB
SUnreclaim:         1712 kB
KernelStack:         384 kB
PageTables:           80 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:        6208 kB
Committed_AS:       5636 kB
VmallocTotal:   262930368 kB
VmallocUsed:         324 kB
VmallocChunk:          0 kB
Percpu:               76 kB
HardwareCorrupted:     0 kB

 cat /proc/zoneinfo 
Node 0, zone   Normal
  per-node stats
      nr_inactive_anon 631
      nr_active_anon 625
      nr_inactive_file 0
      nr_active_file 0
      nr_unevictable 0
      nr_slab_reclaimable 0
      nr_slab_unreclaimable 429
      nr_isolated_anon 0
      nr_isolated_file 0
      workingset_nodes 0
      workingset_refault_anon 0
      workingset_refault_file 0
      workingset_activate_anon 0
      workingset_activate_file 0
      workingset_restore_anon 0
      workingset_restore_file 0
      workingset_nodereclaim 0
      nr_anon_pages 62
      nr_mapped    461
      nr_file_pages 1197
      nr_dirty     0
      nr_writeback 0
      nr_writeback_temp 0
      nr_shmem     1197
      nr_shmem_hugepages 0
      nr_shmem_pmdmapped 0
      nr_file_hugepages 0
      nr_file_pmdmapped 0
      nr_anon_transparent_hugepages 0
      nr_vmscan_write 0
      nr_vmscan_immediate_reclaim 0
      nr_dirtied   0
      nr_written   0
      nr_kernel_misc_reclaimable 0
      nr_foll_pin_acquired 0
      nr_foll_pin_released 0
      nr_kernel_stack 384
  pages free     1162
        min      97
        low      121
        high     145
        spanned  8192
        present  8192
        managed  3104
        protection: (0, 0)
      nr_free_pages 1162
      nr_zone_inactive_anon 631
      nr_zone_active_anon 625
      nr_zone_inactive_file 0
      nr_zone_active_file 0
      nr_zone_unevictable 0
      nr_zone_write_pending 0
      nr_mlock     0
      nr_page_table_pages 19
      nr_bounce    0
      nr_free_cma  0
  pagesets
    cpu: 0
              count: 0
              high:  0
              batch: 1
  vm stats threshold: 2
  node_unreclaimable:  0
  start_pfn:           262144
Node 0, zone  Movable
  pages free     0
        min      0
        low      0
        high     0
        spanned  0
        present  0
        managed  0
        protection: (0, 0)