my battery draining fast, after marshmallow update.

So i installed GSAM battery monitor and saw the processor clock didn't go to a very low state, remaining at ~350khz and that drained the battery. After the october 1, 2016 update (marshmallow version for AT&T on an S5) the clock rate will fairly reliably average ~128khz when the phone is resting (not in doze mode) and significantly improves battery life... for just under 3 days, then the clock rate bumped up to drain the battery faster. I just did a power reset and then it recovered to the lower drain state.
I then got 5 days under light use before getting to 20%. This is pretty much identical to what the phone was doing on lollipop back in january 2016... so 10 months of development on a new OS and they got back to where they started!

So, debugging as much as I can with limited android knowledge. I did find that after a forced update (I can't make it stop) of "galaxy apps" the phone drained ~40% battery overnight when in airplane mode. It seems that something in the galaxy apps wants to update and without a connection to 4G (I don't have a data plan) and no internet, it hangs up and uses processor power that kills the battery. The odd thing is that the last two times I had it draining the battery, GSAM said it was in doze mode... and the processor was at 50% use and .4GHZ.
It's absurd, and I did get the drain to improve after "force close" on the galaxy apps. I then uninstalled that to the factory version and reset the phone again.
I'm backing up now and planning to try a factory reset, then back to lolipop if I can figure out how to download it.

well i can install android 6.0 cuz i have samsung galaxy s4 mini intl with cyanogenmod 12.1 currently but i will install android M (cm 13.0)

i prefer you du battery saver cuz it helped me to use phone cuz it saves battery and saves battery

use du battery saver

well battery saver is good idea but i prefered u du battery saver before but now i prefer go battery pro i now tested it my own

So I can't figure out how to get past the firmware revision lock (ODIN download forces me to stay at marshmallow). I can also not get my bootloader unlocked (tried kingroot, safestrap, towelroot).
What I'm doing - which works very well so far - is to reset the phone every evening. Previous drain events started in the evening (8pm-ish), if I reset my phone at 6pm I have gotten 4 days out of it and I'm at 50% battery... which is just slightly better than it was on lollipop. Works for now, until AT&T / google force another update with new battery drain bugs.
I also noticed that every reboot drops my battery %, so I'm wondering if there's a marshmallow reporting problem, or if it draws more power than usual just to reboot.

Same here. Why does every update seems so unfriendly!

AAH, success! I had previously tried going back to 6.0.1=OCT2016, which is as far as I thought I could go, since I can't root my AT&T S5. I was testing a few things and went back to this version recently, and that initial release had the fewest bugs. I allowed it to update to the 1JAN2017 patch... at which time it said it was up to date (even though I had a February update previously). I also noticed bugs like the power button not turning off the screen (or have it flash back on if it went off).

Long story short, I finally tried going to 5.1.1, April 2016 patch date, which had separate files (AP,BL,CP,CSC), and used ODIN for the download. It allowed me to downgrade back to lollipop with no issues. I had previously tried to go to 4.4.2 as a single file download, which was just too far back... or maybe the separate files is different somehow.
I then promptly installed "no root firewall" by Grey Shirts and disabled all the apps i could... this disabled the OS from downloading updates. There are messages of connecting to the DM server, but it doesn't get any data. (WOO HOO).

Files were from a search for the filename directly and found at uploadgig... /7Aae30D13ae54f6d/G900A_ATT_G900AUCS4CPA6_G900AATT4CPA6.zip
I found the list of filenames and OS level / patch date at galaxys5update.

Try Du battery saver app..

That can do the same the advanced user can do, and it also drains battery (what a paradox).

Is this a common problem for smartphone? Please, i want to update my Huawei Honor 8 to Android 7.0, but worry about the battery life.

In most of cases factory reset should help, but without making backup and restoring data after upgrade.